Create Appearance preference settings

[jandubois]

We should make app appearance configurable:

Theme: (*) System ( ) Light ( ) Dark

System follows the system settings (what we do right now), Light or Dark would always use the specified theme, regardless of system settings. ¹

Accent Color: (*) Rancher Desktop ( ) System ( ) Custom

The "Rancher Desktop" accent color should match the logo color. Custom colour will require a colour picker, similar to the "Branding" option in Rancher Manager.

See #3274 for some more discussion of using accent colours.

These settings could go on their own "Application | Appearance" tag, or be combined with other settings on a "General" tab.

Footnotes

1. We need to decide if using the non-system settings only apply to the window contents, or also to the window title bar, menu, and borders. ↩

[mook-as]

It's unclear to me why we need a mode that doesn't match the system preferences; could you elaborate?

Note that some parts of the UI (e.g. dashboard) isn't directly controlled by us (we can override it if we really want to, probably).

[jandubois]

It's unclear to me why we need a mode that doesn't match the system preferences; could you elaborate?

A user may want to have the Light theme in general, but still prefer the dark theme for RD. Or configure the system to change according to day/night time, but still wants a specific theme for RD all the time.

Same thing for accent colour.

Note that some parts of the UI (e.g. dashboard) isn't directly controlled by us (we can override it if we really want to, probably).

Yes, we can do that. We do this for the screenshots script, but I guess this is now redundant on macOS and Windows, because we also toggle the system theme, so the title and menu bar colors are correct for the theme.