Replace Process with Thread because windows can't
do the fork. Shouldn't change anything on Linux. Also,
replace all the os.seps with '/' just to make sure there's
nothing funny happening with the paths.
Fix css path awkwardness on windows
Don't weird the user out if there's any extensions
they want to use, but don't have installed. Reminder to self:
be sure to flag this in the GUI AND provide a GUI for saving
configurations for each extension just in case users want to
configure those extensions easily.