custom-available-themes is a compiled Lisp function in `
Return a list of Custom themes available for loading.
Search the directories specified by `custom-theme-load-path' for
files named FOO-theme.el, and return a list of FOO symbols.
The returned symbols may not correspond to themes that have been
loaded, and no effort is made to check that the files contain
valid Custom themes. For a list of loaded themes, check the