Fixes#30895.
The root font-size for the app has always been 16px, and it remains the
same when switching between 14px and 16px mode. This leads to
`.small` buttons looking relatively smaller to their surroundings in
16px mode. So, if we use a unit that changes when switching between
modes, we will have to multiple that value by (16/14) so that we don't
change the existing behaviour for the 14px mode and make things smaller.
This commit only affects `.small` buttons, the same named class is used
in other places but does not use rem unit there and therefore does not
require any changes.
The original plan was to use em instead of rem and multiply it by
(16/14 * 0.8), but since em depends on the parent element, there was a
case in the poll button widget where 1em was equal to 16px in the dense
14px mode. While, the right thing to do might have been some refactor to
make it work as desired, the safest thing to do right now might be to
use the --base-font-size-px variable directly for the 9.0 release.
15ea879899