tab: Use the default flat style for buttons

4 jobs for wip/exalm/tab-fixes in 7 minutes and 51 seconds
Status Job ID Name Coverage
  Build
passed #1341039
abi-check

00:03:12

passed #1341036
api-visibility

00:00:13

passed #1341038
flatpak
build-flatpak

00:07:51

passed #1341037
flatpak
doc

00:02:32