diff --git a/docs/guide/CSS.md b/docs/guide/CSS.md
index 2e27db2ca9..499f53a50b 100644
--- a/docs/guide/CSS.md
+++ b/docs/guide/CSS.md
@@ -327,7 +327,7 @@ The `background: green` is only applied to the Button underneath the mouse curso
 Here are some other pseudo classes:
 
 - `:blur` Matches widgets which *do not* have input focus.
-- `:dark` Matches widgets in dark mode (where `App.dark == True`).
+- `:dark` Matches widgets in dark themes (where `App.theme.dark == True`).
 - `:disabled` Matches widgets which are in a disabled state.
 - `:enabled` Matches widgets which are in an enabled state.
 - `:even` Matches a widget at an evenly numbered position within its siblings.
@@ -336,7 +336,7 @@ Here are some other pseudo classes:
 - `:focus` Matches widgets which have input focus.
 - `:inline` Matches widgets when the app is running in inline mode.
 - `:last-of-type` Matches a widget that is the last of its type amongst its siblings.
-- `:light` Matches widgets in dark mode (where `App.dark == False`).
+- `:light` Matches widgets in light themes (where `App.theme.dark == False`).
 - `:odd` Matches a widget at an oddly numbered position within its siblings.
 
 ## Combinators
diff --git a/examples/theme_sandbox.py b/examples/theme_sandbox.py
index 07513f08a6..9445c634ae 100644
--- a/examples/theme_sandbox.py
+++ b/examples/theme_sandbox.py
@@ -248,7 +248,7 @@ def watch_theme(self, theme_name: str) -> None:
 
     def compose(self) -> ComposeResult:
         with Grid(id="palette"):
-            theme = self.get_theme(self.theme)
+            theme = self.current_theme
             for variable, value in vars(theme).items():
                 if variable not in {
                     "name",
diff --git a/src/textual/_text_area_theme.py b/src/textual/_text_area_theme.py
index 0373136bba..e315716f30 100644
--- a/src/textual/_text_area_theme.py
+++ b/src/textual/_text_area_theme.py
@@ -90,8 +90,7 @@ def apply_css(self, text_area: TextArea) -> None:
         if self.base_style.color is None:
             self.base_style = Style(color="#f3f3f3", bgcolor=self.base_style.bgcolor)
 
-        app = text_area.app
-        app_theme = app.get_theme(app.theme)
+        app_theme = text_area.app.current_theme
 
         if self.base_style.bgcolor is None:
             self.base_style = Style(
diff --git a/src/textual/app.py b/src/textual/app.py
index 8e90ba5cc7..20f4aa6ed9 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -558,7 +558,9 @@ def __init__(
         
         This excludes the built-in themes."""
 
-        ansi_theme = self.ansi_theme_dark if self.dark else self.ansi_theme_light
+        ansi_theme = (
+            self.ansi_theme_dark if self.current_theme.dark else self.ansi_theme_light
+        )
         self.set_reactive(App.ansi_color, ansi_color)
         self._filters: list[LineFilter] = [
             ANSIToTruecolor(ansi_theme, enabled=not ansi_color)
@@ -764,8 +766,8 @@ def __init__(
         perform work after the app has resumed.
         """
 
-        self.set_class(self.dark, "-dark-mode")
-        self.set_class(not self.dark, "-light-mode")
+        self.set_class(self.current_theme.dark, "-dark-mode")
+        self.set_class(not self.current_theme.dark, "-light-mode")
 
         self.animation_level: AnimationLevel = constants.TEXTUAL_ANIMATIONS
         """Determines what type of animations the app will display.
@@ -1183,11 +1185,7 @@ def get_css_variables(self) -> dict[str, str]:
         Returns:
             A mapping of variable name to value.
         """
-        theme = self.get_theme(self.theme)
-        if theme is None:
-            # This should never happen thanks to the `App.theme` validator.
-            return {}
-
+        theme = self.current_theme
         # Build the Textual color system from the theme.
         # This will contain $secondary, $primary, $background, etc.
         variables = theme.to_color_system().generate()
@@ -1265,28 +1263,13 @@ def _watch_theme(self, theme_name: str) -> None:
         self.call_next(self.refresh_css)
         self.call_next(self.theme_changed_signal.publish, theme)
 
-    def watch_dark(self, dark: bool) -> None:
-        """Watches the dark bool.
-
-        This method handles the transition between light and dark mode when you
-        change the [dark][textual.app.App.dark] attribute.
-        """
-        self.set_class(dark, "-dark-mode", update=False)
-        self.set_class(not dark, "-light-mode", update=False)
-        self._refresh_truecolor_filter(self.ansi_theme)
-        self.call_next(self.refresh_css)
-
     def watch_ansi_theme_dark(self, theme: TerminalTheme) -> None:
-        app_theme = self.get_theme(self.theme)
-        assert app_theme is not None  # validated by _validate_theme
-        if app_theme.dark:
+        if self.current_theme.dark:
             self._refresh_truecolor_filter(theme)
             self.call_next(self.refresh_css)
 
     def watch_ansi_theme_light(self, theme: TerminalTheme) -> None:
-        app_theme = self.get_theme(self.theme)
-        assert app_theme is not None  # validated by _validate_theme
-        if not app_theme.dark:
+        if not self.current_theme.dark:
             self._refresh_truecolor_filter(theme)
             self.call_next(self.refresh_css)
 
@@ -1297,9 +1280,9 @@ def ansi_theme(self) -> TerminalTheme:
         Defines how colors defined as ANSI (e.g. `magenta`) inside Rich renderables
         are mapped to hex codes.
         """
-        app_theme = self.get_theme(self.theme)
-        assert app_theme is not None  # validated by _validate_theme
-        return self.ansi_theme_dark if app_theme.dark else self.ansi_theme_light
+        return (
+            self.ansi_theme_dark if self.current_theme.dark else self.ansi_theme_light
+        )
 
     def _refresh_truecolor_filter(self, theme: TerminalTheme) -> None:
         """Update the ANSI to Truecolor filter, if available, with a new theme mapping.
diff --git a/src/textual/dom.py b/src/textual/dom.py
index 89be69a9fb..d0b9bb4723 100644
--- a/src/textual/dom.py
+++ b/src/textual/dom.py
@@ -237,7 +237,7 @@ def set_reactive(
 
         Example:
             ```python
-            self.set_reactive(App.dark_mode, True)
+            self.set_reactive(App.theme, "textual-light")
             ```
 
         Args:
@@ -247,15 +247,14 @@ def set_reactive(
         Raises:
             AttributeError: If the first argument is not a reactive.
         """
+        name = reactive.name
         if not isinstance(reactive, Reactive):
-            raise TypeError(
-                "A Reactive class is required; for example: MyApp.dark_mode"
-            )
-        if reactive.name not in self._reactives:
+            raise TypeError("A Reactive class is required; for example: MyApp.theme")
+        if name not in self._reactives:
             raise AttributeError(
-                "No reactive called {name!r}; Have you called super().__init__(...) in the {self.__class__.__name__} constructor?"
+                f"No reactive called {name!r}; Have you called super().__init__(...) in the {self.__class__.__name__} constructor?"
             )
-        setattr(self, f"_reactive_{reactive.name}", value)
+        setattr(self, f"_reactive_{name}", value)
 
     def mutate_reactive(self, reactive: Reactive[ReactiveType]) -> None:
         """Force an update to a mutable reactive.
@@ -1218,11 +1217,11 @@ def watch(
 
         Example:
             ```python
-            def on_dark_change(old_value:bool, new_value:bool) -> None:
-                # Called when app.dark changes.
-                print("App.dark went from {old_value} to {new_value}")
+            def on_theme_change(old_value:str, new_value:str) -> None:
+                # Called when app.theme changes.
+                print(f"App.theme went from {old_value} to {new_value}")
 
-            self.watch(self.app, "dark", self.on_dark_change, init=False)
+            self.watch(self.app, "theme", self.on_theme_change, init=False)
             ```
 
         Args:
diff --git a/src/textual/widget.py b/src/textual/widget.py
index 585318919c..48427282c8 100644
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -375,8 +375,8 @@ class Widget(DOMNode):
         "can-focus": lambda widget: widget.can_focus,
         "disabled": lambda widget: widget.is_disabled,
         "enabled": lambda widget: not widget.is_disabled,
-        "dark": lambda widget: widget.app.dark,
-        "light": lambda widget: not widget.app.dark,
+        "dark": lambda widget: widget.app.current_theme.dark,
+        "light": lambda widget: not widget.app.current_theme.dark,
         "focus-within": lambda widget: widget.has_focus_within,
         "inline": lambda widget: widget.app.is_inline,
         "ansi": lambda widget: widget.app.ansi_color,
diff --git a/src/textual/widgets/_markdown.py b/src/textual/widgets/_markdown.py
index 694c14944d..ee4ddd00b7 100644
--- a/src/textual/widgets/_markdown.py
+++ b/src/textual/widgets/_markdown.py
@@ -619,10 +619,9 @@ def __init__(self, markdown: Markdown, code: str, lexer: str) -> None:
         super().__init__(markdown)
         self.code = code
         self.lexer = lexer
-        app_theme = self.app.get_theme(self.app.theme)
         self.theme = (
             self._markdown.code_dark_theme
-            if app_theme.dark
+            if self.app.current_theme.dark
             else self._markdown.code_light_theme
         )
 
@@ -642,10 +641,9 @@ def _on_mount(self, _: Mount) -> None:
 
     def _retheme(self) -> None:
         """Rerender when the theme changes."""
-        app_theme = self.app.get_theme(self.app.theme)
         self.theme = (
             self._markdown.code_dark_theme
-            if app_theme.dark
+            if self.app.current_theme.dark
             else self._markdown.code_light_theme
         )
         self.get_child_by_type(Static).update(self._block())
@@ -807,13 +805,13 @@ async def _on_mount(self, _: Mount) -> None:
 
     def _watch_code_dark_theme(self) -> None:
         """React to the dark theme being changed."""
-        if self.app.dark:
+        if self.app.current_theme.dark:
             for block in self.query(MarkdownFence):
                 block._retheme()
 
     def _watch_code_light_theme(self) -> None:
         """React to the light theme being changed."""
-        if not self.app.dark:
+        if not self.app.current_theme.dark:
             for block in self.query(MarkdownFence):
                 block._retheme()
 
diff --git a/src/textual/widgets/_text_area.py b/src/textual/widgets/_text_area.py
index 44fd681554..63d2d95a61 100644
--- a/src/textual/widgets/_text_area.py
+++ b/src/textual/widgets/_text_area.py
@@ -745,7 +745,7 @@ def _watch_theme(self, theme: str) -> None:
         if padding is applied, the colors match."""
         self._set_theme(theme)
 
-    def _app_dark_toggled(self) -> None:
+    def _app_theme_changed(self) -> None:
         self._set_theme(self._theme.name)
 
     def _set_theme(self, theme: str) -> None:
@@ -1521,8 +1521,8 @@ def gutter_width(self) -> int:
         return gutter_width
 
     def _on_mount(self, event: events.Mount) -> None:
-        # When `app.dark` is toggled, reset the theme (since it caches values).
-        self.watch(self.app, "dark", self._app_dark_toggled, init=False)
+        # When `app.theme` reactive is changed, reset the theme to clear cached styles.
+        self.watch(self.app, "theme", self._app_theme_changed, init=False)
 
         self.blink_timer = self.set_interval(
             0.5,