theme: Fix missing text on virtual keyboard

Use em to set the button size. This allows the size to adjust when the
keyboard size is changed. Also fix the boxpointer menu that holds the
subkeys.

Fixes: linuxmint/mint22.1-beta#25

data/theme/cinnamon-sass/widgets/_keyboard.scss

@@ -13,8 +13,9 @@
   &-key {
     @extend %button;
 
-    min-height: 30px;
-    min-width: 30px;
+    min-height: 2em;
+    min-width: 2em;
+    padding: 0;
     font-size: 1.2em;
     font-weight: bold;
 
@@ -23,7 +24,10 @@
 
   &-subkeys {
     padding: $base_padding;
-    -arrow-border-radius: $borders_color;
+    -arrow-background-color: $bg_color;
+    -arrow-border-width: 1px;
+    -arrow-border-radius: $base_border_radius;
+    -arrow-border-color: lighten($borders_color, 10%);
     -arrow-base: 20px;
     -arrow-rise: 10px;
     -boxpointer-gap: 0;