Fix regression from https://github.com/go-gitea/gitea/pull/30033
These buttons had lost their border because `.ui.header` sets `none` but
`.ui.menu` has it, after the migration, the order of styles changed and
header won. I see no reason why those have the `header` class in first
place, besides for semantic meaning.
Before:
<img width="491" alt="Screenshot 2024-03-25 at 00 39 27"
src="fa1b7505-75cf-4854-a97f-db3c46f31e93">
After:
<img width="496" alt="Screenshot 2024-03-25 at 00 39 14"
src="8f6bdc07-9596-436b-8c82-9af283300004">
(cherry picked from commit 8717c1c2bef1afcc6b0bb2d84627b158b95836b0)
67659c5d46