Add Light Theme for Web UI

[arobinsongit]

As an older developer the dark theme (gray on black) can be very difficult for me to read.

Adding the ability to change the web UX to a light theme would be very helpful. The light theme on the doc page (https://docs.netbird.io/) is perfectly readable and would be a good option.

I have noticed in the top right you have something that appears to be a theme selector button but both Light and System are disabled? Maybe this is in the works?