software/theia: backport a CSS fix, to be removed next time we update

Without this fix, selecting a completion with the keyboard is not really
possible.
2 jobs for feat/theia-1.18.0 in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

06:13:58

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

08:08:27