Don't apply window_decorations.css to non-Breeze themes.

Some applications might want to switch GTK themes dynamically, for a
variety of reasons.

For example, Firefox Nightly recently started doing this, so that
Firefox theme users have a better experience if they choose a Firefox
theme different from their GTK theme:

 * https://bugzilla.mozilla.org/show_bug.cgi?id=1707872

However, this causes some issues in KDE. While we usually try to switch
to the most compatible theme, we might fall back to Adwaita Dark. In
such case, this CSS would still apply causing artifacts as shown here:

 * https://bugzilla.mozilla.org/show_bug.cgi?id=1716285

Listen to the theme dynamically to make sure we don't apply
window_decorations.css in such case.

I plan to figure out how to work around this in Firefox / figure out why
Breeze dark is not working properly, but this seems worth it anyways.