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/strings.xml

@@ -196,6 +196,7 @@
     <string name="action_links">Links</string>
     <string name="action_mentions">Mentions</string>
     <string name="action_hashtags">Hashtags</string>
+    <string name="action_follow_hashtag">Follow a new hashtag</string>
     <string name="action_open_reblogger">Open boost author</string>
     <string name="action_open_reblogged_by">Show boosts</string>
     <string name="action_open_faved_by">Show favorites</string>