Bug 1876719 - [a11y] Increasing contrast for collection's dialog buttons

Before the fix, Accessibility Scanner was showing the color contrast
between text color and the background color as insufficient.

  With the modification made the accessibility scanner shows the
problem as fixed. The color that was used for the fix is the closest to
the one before that can satisfy the contrast required.

app/src/main/res/values-night/colors.xml

@@ -150,7 +150,7 @@
 
     <!-- Normal theme colors for dark mode -->
     <color name="accent_normal_theme">@color/photonViolet50</color>
-    <color name="accent_high_contrast_normal_theme">@color/photonViolet40</color>
+    <color name="accent_high_contrast_normal_theme">@color/photonViolet20</color>
     <color name="neutral_normal_theme">@color/photonGrey20</color>
     <color name="neutral_faded_normal_theme">#1FFBFBFE</color>
     <color name="toggle_off_knob_normal_theme">@color/toggle_off_knob_dark_theme</color>