Add theme manager to ide

[TomasRoj]

it would be good to have such a theme manager in ide. When I firstly downloaded the ide I was suprised that only one theme was availibe there. @per1234 has nice ino-themes project that can be used for example. Thanks, Tom.

[per1234]

A theme manager was already added by #7124:

I just haven't gotten around to updating the installation instructions in my theme repository yet for that installation option as well as the one added by #7115.

The tricky thing about the theme installation system added in #7124 is that you need to structure your .zip files so that the theme is in the root of the .zip file, not in a subfolder. For this reason. you can't just download the .zip files from GitHub and dump them right into the theme subfolder of your sketchbook folder. You need to manually create the .zip files with the correct structure.