https://www.phpbb.com/customise/db/styl ... r/demo/3.2
there are a few that fit your criteria in that demo. also, you can always make any changes to a style that you wish.
you open the style files in a good text editor and make your wanted changes. then save the file and upload it to the server where it belongs.
then you have to purge the cache after every change and refresh your browser to see those changes.