So first, we only provided the prosilver xsl templates so far, so it will be easier for prosilver based theme.
Then, the principle.
The files involved for the layout are gym_google.xsl, gym_rss.xsl and gym_js.js. They are both in the gym_sitemaps/style/ and the style/prosilver/template/ dirs.
These files goes with css file, gym_google.css and gym_rss.css, located both in gym_sitemaps/style/ and the style/prosilver/theme/ dirs.
These are indeed templates, like phpBB templates and css, they need to be parsed to be used in the display, which is done by the gym_sitemaps/gym_style.php file kind of like the way the phpBB style.php file will do, but both for css and xsl and without the need to connect to the db each time, if the file is cached, it will be sent as is directly.
The xsl and css cached files goes in gym_sitemaps/cache/.
So talking about admintiger's case, a theme based on prosilver, try to copy the css, js and xsl files in the corresponding dirs of your theme. Then, go ACP and activate the use of phpBB's css, go to the maintenance menu and clear the outptu cache).
The xsl prosilver template should be now displayed with your theme's css, thus colors and background imgages.
If you change the html layout in the templates, you may need to do similar changes in the xsl files, but chances are great that it will work nicely directly on the xhtml code.
You could as well not load the phpBB's css and mod the gym css files to match your theme, but I think that's it's easier to eventually do few changes in the xsl file than to re do the whole css.
Then, if you want to do the same thing with a theme based on subsilver2 or totally different, the principle is the same, but you'll most likely need to implement more change in the xsl files to match your theme html code and css class names better.
With time, we'l provide with more theme's xsl files, css are less important, since you can use the phpBB one directly. With a supported theme like prosilver it's very cool, all your personalisations gets taken into account directly in the sitemaps and rss feeds as long as you use the phpbb's css (one click in acp)