tools (common/template): theme settings with possibility to disable default fallback for CSS: - themes can now have a `settings.json` file at their root to modify the renderer behaviour - the `css_default_fallback` boolean (true by default) can be used to disable the fallback mechanism when CSS files are checked. Useful when a theme doesn't follow the CSS naming of default theme. - introduced some type hints

You'll find install instructions in documentation (see doc/installation.rst).

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