Fix resource picker button style

[Giganzo]

Fixes: #112232

Before:

After:

Screencast_20251031_132521.webm

This change has minimal effect on the classic theme

4.6.dev2:

After:

[Mickeon]

While I do approve of this PR because it's just as it was before, would it be completely weird and inconsistent to ask for these kinds of buttons to feature a more prominent background, but not brighter than the Inspector's itself?

[CardboardCarl]

While I do approve of this PR because it's just as it was before, would it be completely weird and inconsistent to ask for these kinds of buttons to feature a more prominent background, but not brighter than the Inspector's itself?

@Mickeon I think that might be better off as its own ticket; I'm concerned adding it might muddy the intent of this PR too much.

If I'm following correctly, the purpose of this PR was just to change the buttons from the "primary" style to the "flat" style. It seems they've been primary buttons this whole time when they should've been flat to begin with in order to match the design.

[akien-mga]

Thanks!