Add a style for .keycap (#171)

Author: danirabbit

gtk-3.22/gtk-widgets.css

@@ -138,6 +138,28 @@ rubberband,
     font-family: monospace;
 }
 
+.keycap {
+    background-image:
+        linear-gradient(
+            to bottom,
+            shade (
+                @bg_color,
+                0.95
+            ),
+            @bg_color
+        );
+    border: 1px solid @border_color;
+    border-radius: 2.5px;
+    box-shadow:
+        inset 0 0 0 1px alpha (@bg_highlight_color, 0.05),
+        inset 0 1px 0 0 alpha (@bg_highlight_color, 0.45),
+        inset 0 -1px 0 0 alpha (@bg_highlight_color, 0.15),
+        0 1px 0 0 @border_color;
+    min-width: 16px;
+    padding: 3px 6px;
+    text-shadow: 0 1px @text_shadow_color;
+}
+
 /**************
  * Separators *
  *************/