The files in the ui directory are:
This is really just a hub file that points to the three files that drive S5. Those files are:
Contains a few rules that make the system run. If you mess with this file, you're likely to break the slide show unless you really know what you're doing.
Contains some rules that place and size the various slide components—things like the header, footer, slide controls, and so on. If you want to change the basic slide layout, this is where to go.
The CSS that adds the visual pizazz to the slide show—colors, fonts, text alignment, margins, and all that other good stuff. You can make a whole new theme just by editing this file, and not touching the other two.
This file is what allows Opera Show to do its thing, assigning a few CSS rules that break the file up into slides. It should never have to be edited unless you go messing with the rules in S5-core.css (not recommended, but hey, it's your slide show).
This is what makes the printer-friendly version work. There are a couple of rules at the beginning of the file you should be careful about changing, but otherwise, it's all up to you.