Add white outline to black emojis (fix #5083) (#7936)

app/javascript/styles/application.scss

@@ -22,3 +22,4 @@
 @import 'mastodon/tables';
 @import 'mastodon/admin';
 @import 'mastodon/rtl';
+@import 'mastodon/accessibility';

app/javascript/styles/mastodon/accessibility.scss

@@ -0,0 +1,14 @@
+$black-emojis: '8ball' 'ant' 'back' 'black_circle' 'black_large_square' 'black_medium_small_square' 'black_medium_square' 'black_nib' 'black_small_square' 'bomb' 'bust_in_silhouette' 'camera' 'camera_with_flash' 'clubs' 'copyright' 'curly_loop' 'currency_exchange' 'end' 'heavy_check_mark' 'heavy_division_sign' 'heavy_dollar_sign' 'heavy_minus_sign' 'heavy_multiplication_x' 'heavy_plus_sign' 'lower_left_fountain_pen' 'on' 'registered' 'soon' 'spades' 'spider' 'tm' 'top' 'waving_black_flag' 'wavy_dash' 'video_game';
+
+%white-emoji-outline {
+  filter: drop-shadow(1px 1px 0 $white) drop-shadow(-1px 1px 0 $white) drop-shadow(1px -1px 0 $white) drop-shadow(-1px -1px 0 $white);
+  transform: scale(.71);
+}
+
+.emojione {
+  @each $emoji in $black-emojis {
+    &[title=':#{$emoji}:'] {
+      @extend %white-emoji-outline;
+    }
+  }
+}