Override focus and colour CSS button styles

Set all button focus outline properties and set the colour and filter
on fixed buttons so more theme styles are overridden.

src/alsa-scarlett-gui.css

@@ -83,7 +83,7 @@ label.gain {
 }
 
 .window-frame button:focus:focus-visible {
-  outline-color: #801010;
+  outline: 2px solid #801010;
 }
 
 /* padding doesn't work when selected with .window-frame, so use
@@ -118,6 +118,11 @@ button.toggle {
   filter: none;
 }
 
+.window-frame button.fixed label {
+  color: #ffffff;
+  filter: none;
+}
+
 /* Combobox controls that are always disabled because they indicate status */
 .window-frame combobox.fixed > box > button {
   color: #ffffff;