3477 tweak colors for contrast (#3921)

Fixes: #3477

This tweaks two colors: 
* for the dark theme a new slightly darker (timeline) background is
used.
* for the light theme a darker shade of blue is used as highlight color
(`colorPrimary` and `colorSecondary`)

Additional change:
* Use green and red more consistent: use existing colors for the diff
display; move the new light variants to colors.xml like all colors

app/src/main/java/com/keylesspalace/tusky/components/compose/ComposeActivity.kt

@@ -1058,7 +1058,7 @@ class ComposeActivity :
         binding.composeCharactersLeftView.text = String.format(Locale.getDefault(), "%d", remainingLength)
 
         val textColor = if (remainingLength < 0) {
-            getColor(R.color.tusky_red)
+            getColor(R.color.warning_color)
         } else {
             MaterialColors.getColor(
                 binding.composeCharactersLeftView,