style: Hide border on carousel buttons for non-Adwaita themes

This makes no difference to the carousel appearance using the default
theme (Adwaita), but should fix some glitches with other themes.

Investigated by Milan Crha.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #1159
2 jobs for 1159-carousel-button-other-theme in 6 minutes and 37 seconds (queued for 1 second)