Add a button-style for subtle highlighting

and use it to subtly highlight commonly used symbols in the
accent-views of the German layouts.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/637>

data/style.css

@@ -14,6 +14,14 @@ sq_button:active {
     background: alpha(@theme_fg_color, 0.11);
 }
 
+sq_button.subtle-highlight {
+    background: alpha(@theme_fg_color, 0.105);
+}
+
+sq_button.subtle-highlight:active {
+    background: alpha(@theme_fg_color, 0.16);
+}
+
 sq_button.emoji-group {
     background: alpha(@theme_fg_color, 0.105);
     border-bottom: 0.4999px;