tools (common/template): new `fallback` settings: `fallback` can be used to change fallback behaviour. By default, fallback is done on `default` theme, this can be set to an other theme with a string, or to a list of fallback. The list can also be empty is no fallback is desired (notably usefull for "main" themes, on which other themes may fallback). This setting replaces `css_default_fallback`.