tools (common): helping method to calculate luminance: - a new `utils` module has been added for backend and frontends generic utilities - per_luminance method calculate the perceived luminance of a colour (is it dark or light?). This is useful to automatically select the suitable colour in a theme.

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

If you need help, come the SàT official XMPP room: sat@chat.jabberfr.org or join us with a web browser via https://chat.jabberfr.org/converse.js/sat@chat.jabberfr.org