better selection color contrast for black theme

css/theme/source/black.scss

@@ -30,7 +30,7 @@ $headingTextTransform: uppercase;
 $headingFontWeight: 600;
 $linkColor: #42affa;
 $linkColorHover: lighten( $linkColor, 15% );
-$selectionBackgroundColor: lighten( $linkColor, 25% );
+$selectionBackgroundColor: rgba( $linkColor, 0.75 );
 
 $heading1Size: 2.5em;
 $heading2Size: 1.6em;

dist/theme/black.css

@@ -34,7 +34,7 @@ section.has-light-background, section.has-light-background h1, section.has-light
   --r-link-color: #42affa;
   --r-link-color-dark: #068de9;
   --r-link-color-hover: #8dcffc;
-  --r-selection-background-color: #bee4fd;
+  --r-selection-background-color: rgba(66, 175, 250, 0.75);
   --r-selection-color: #fff;
 }