UX: Fix theme upload width, remove class clash, prettier (PR #13071)

Minor class clash here, the composer file modal & theme upload modal used to be similar… but the theme uploader has diverged, so I’m removing the old class to fix this issue:

Before:

After:

GitHub

Looks like your text editor did you dirty and ran prettier on the template?

@cvx ok fixed! the extension updated to the latest prettier, which supports hbs formatting! so it was a little premature.

I am marking this for backporting, nice change!