change Bootstrap theme via config

[Bergruebe]

Hey, it would be great if the Bootstrap theme would be configurable in the config file. I am a dark mode fan, so I change the theme after every update to a dark theme manually, because my changes in index.html are overwritten.

So it would be really great if thees lines would be accessible over the config file: https://github.com/Luzifer/nginx-sso/blob/58021f2cfe0727d3d6f923be2cd20b632700e2c9/frontend/index.html#L10-L12

as an optional argument. In this way it would be also possible to self host the theme so you don't depend on jsdelivr for the login page. (I needed some time to find out, that I blocked jsdelivr in PiHole by mistake.)

Thank you very much for your work and this great pice of software, which protect my personal web apps. Danke!