Wrong permissions for style CSS file

Commit f87317d0 uses:

os.chmod(css_file_name, stat.S_IWRITE)

Which sets the permissions to write-only for the user. This prevents things like reading the file, or copying it, or moving it to a new location.

This breaks the build of modules using gtk-doc on tools that install to a separate DESTDIR and then move the files to the appropriate location, like:

• jhbuild
• Buildstream
• various Linux distribution packaging tools

The flags should, at the very least, be stat.S_IWRITE | stat.S_IREAD, but I'd recommend using 0644.