Enhancement: Theme Manager

*Warning: This pullrequest is only opened for deeper testing.*

What does this change? It ref. #422 and resolves #407 .

Currently, this pull should *not* be merged, but as I can not find any issue, I would be happy if someone else could also test it and share me your opinion. Themes can be changed during the login and the default one is defined in config.php as well. Selected theme will be stored in localstorage and assigned on each login as long as the theme is activated.