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/layout/activity_tab_preference.xml

@@ -29,7 +29,8 @@
         android:layout_gravity="bottom|end"
         android:layout_margin="16dp"
         android:contentDescription="@string/action_add_tab"
-        app:srcCompat="@drawable/ic_plus_24dp" />
+        app:srcCompat="@drawable/ic_plus_24dp"
+        app:tint="?attr/colorOnPrimary" />
 
     <com.google.android.material.card.MaterialCardView
         android:id="@+id/sheet"
@@ -66,7 +67,8 @@
                 android:text="@string/action_add_tab"
                 android:textColor="?attr/colorOnPrimary"
                 android:textSize="?attr/status_text_large"
-                app:drawableStartCompat="@drawable/ic_plus_24dp" />
+                app:drawableStartCompat="@drawable/ic_plus_24dp"
+                app:drawableTint="?attr/colorOnPrimary" />
         </LinearLayout>
 
     </com.google.android.material.card.MaterialCardView>