improved color scheme & theming (#4507)

Follow up to https://github.com/tuskyapp/Tusky/pull/3921

- no more hardcoded `tusky_blue`, instead the `colorPrimary` attribute
is used. This will help us when adding more themes, e.g a dynamic color
one.
- The `colorPrimary` of the dark theme is now lighter for more contrast
and subsequently the `colorOnPrimary` is now dark grey instead of white.
- `tusky_red_lighter` is now a bit more red than before
- Tweaked color usage in a few places for better contrast

I think this looks a bit unfamiliar but overall better and the higher
contrast makes things noticeably easier to read.

<img
src="https://github.com/tuskyapp/Tusky/assets/10157047/4cbb92d8-b772-4e94-bc15-c4baf0e5473f"
width="260"/>

app/src/main/res/values/styles.xml

@@ -48,7 +48,7 @@
         <item name="status_text_large">18sp</item>
 
         <item name="colorPrimary">@color/colorPrimary</item>
-        <item name="colorOnPrimary">@color/white</item>
+        <item name="colorOnPrimary">@color/colorOnPrimary</item>
 
         <item name="colorSecondary">@color/colorSecondary</item>
         <item name="colorOnSecondary">@color/white</item>
@@ -93,6 +93,8 @@
     <style name="TuskyBlackThemeBase" parent="TuskyBaseTheme">
         <item name="colorPrimaryDark">@color/tusky_grey_05</item>
 
+        <item name="colorOnPrimary">@color/black</item>
+
         <item name="android:colorBackground">@color/black</item>
         <item name="windowBackgroundColor">@color/black</item>