Add detailed options for the default theme

[ostasevych]

After updating to 28.0.5 I found that NC tries to guess the color of icons and the color of text by default. In fact, I have black text and icons on blue, what is not so easy to read and perceive. Personally, I can fix that, but cannot propagate that for all users.

Please, add the options to define color of icons (black or white) and color of text of icons and buttons (black or white) in the theming settings of admin and user settings as well. This will allow an admin and a user to adjust the theming.