[TASK] Improve styling of sys_action "Create Backend User"

The styling of sys_action "Create Backend User" is outdated
and should be slightly improved to enhance usability of
this module.

Releases: 6.2
Resolves: #59338
Change-Id: Ifaba21a78687589af9ba0decad8bc2b9fdc4d29d
Reviewed-on: https://review.typo3.org/30646
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers