Fix button layout in ThemePreview to match the actual themed-banner:
wrap buttons in spans to mimic custom element wrappers (preventing
flex: 1 from stretching them), use correct padding/background/max-width,
and set explicit line-height: normal. Also clean up dead flex properties
in themed-banner styles.
Signed-off-by: Émile Ré <emile@getprobo.com>
9510d91c61