Each subdirectory under this folder describes a built-in "skin". There are four files in each subdirectory for the CSS, the "details" file, the footer, and the header for that skin.
To improve an existing built-in skin, simply edit the appropriate files and recompile.
To add a new skin:
Create a new subdirectory under skins/. (The new directory is called "skins/newskin" below but you should use a new original name, of course.)
Add files skins/newskin/css.txt, skins/newskin/details.txt, skins/newskin/footer.txt and skins/newskin/header.txt. Be sure to "fossil add" these files.
Go to the src/ directory and rerun "tclsh makemake.tcl". This step rebuilds the various makefiles so that they have dependencies on the skin files you just installed.
Edit the BuiltinSkin array near the top of the src/skins.c source file so that it describes and references the "newskin" skin.
Type "make" to rebuild.
One way to develop a new skin is to copy the baseline files (css.txt, details.txt, footer.txt, and header.txt) into a working directory $WORKDIR then launch Fossil with a command-line option "--skin $WORKDIR". Example:
cp -r skins/default newskin fossil ui --skin ./newskin
When the argument to --skin contains one or more '/' characters, the appropriate skin files are read from disk from the directory specified. So after launching fossil as shown above, you can edit the newskin/css.txt, newskin/details.txt, newskin/footer.txt, and newskin/header.txt files using your favorite text editor, then press Reload on your browser to see immediate results.