data/style.css: Highlight "wide"-outline

The "wide"-outline was not highlighted on the default and high
contrast styles, while it was highlighted on the "Adwaita:dark"-style.
"Adwaita:dark" is the default on Phosh, so adjust to that, for
consistent button-highlighting.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/634>

data/style.css

@@ -15,12 +15,14 @@ sq_button:active {
 }
 
 sq_button.altline,
-sq_button.special {
+sq_button.special,
+sq_button.wide {
     background: alpha(@theme_fg_color, 0.15); 
 }
 
 sq_button.altline:active,
-sq_button.special:active {
+sq_button.special:active,
+sq_button.wide:active {
     background: alpha(@theme_fg_color, 0.2);
 }