Style colour card section

This may make more sense in the colour-card.css file but as it was
styling the top level component I put it in the general css file.
I think it's fine that the colour-cards file doesn't use the CSS vars as
it's a seperate component, but it makes sense for all the components in
this file to.

client/assets/styles/main.css

@@ -159,3 +159,18 @@ main {
   align-self: center;
   width: var(--spacing-xx-large);
 }
+
+#colour-section {
+  width: 30.5rem;
+  margin-bottom: var(--spacing-base);
+
+  fieldset {
+    margin-bottom: var(--spacing-base);
+  }
+
+  h2 {
+    margin-bottom: var(--spacing-medium-small);
+  }
+}
+
+