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/java/com/keylesspalace/tusky/util/LinkHelper.kt

@@ -43,6 +43,7 @@ import androidx.browser.customtabs.CustomTabsIntent
 import androidx.core.net.toUri
 import androidx.preference.PreferenceManager
 import at.connyduck.sparkbutton.helpers.Utils
+import com.google.android.material.R as materialR
 import com.google.android.material.color.MaterialColors
 import com.keylesspalace.tusky.R
 import com.keylesspalace.tusky.entity.HashTag
@@ -329,7 +330,7 @@ private fun openLinkInBrowser(uri: Uri?, context: Context) {
 fun openLinkInCustomTab(uri: Uri, context: Context) {
     val toolbarColor = MaterialColors.getColor(
         context,
-        com.google.android.material.R.attr.colorSurface,
+        materialR.attr.colorSurface,
         Color.BLACK
     )
     val navigationbarColor = MaterialColors.getColor(