I wanted different colors and fonts for the stardust theme in chyrp.
Plan A
I edited /themes/stardust/stylesheets/screen.css. This worked but screen.css ended up a bit of a mess and I had trouble tracking some elements down.
Plan B
I put my edits in a new file /themes/stardust/stylesheets/screen.neilt.css. I used the excellent Inspect Element facility in Safari to find out the styles for elements. I then put the styles for just those elements that I wanted to change in screen.neilt.css. This works because chyrp loads all the style sheets it finds in the stylesheets folder. Chyrp must of course load them in the correct order for my styles take priority over previous styles. I am not sure of the rules that govern the load order but alphabetic seems to work for this situation.