xhtml_style: adjust CSS rule for dialog actions margin

This updates the change to 32cead86 (xhtml_style: set a bottom
margin for dialog action, 2019-05-31) that got impacted by a rule
added on html and body in 5dde17a5 (xhtml_style: make editors in
bottom group use full height, 2022-08-22)
4 jobs for feat/monaco_0.34.1 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:25

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:34

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:32

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:43