This will not break your panel unless you use a diffrent theme version for a diffrent panel version, if it for some reason does break your panel it is simple to remove the theme.
Make sure that the theme you are installing corresponds with the version of the panel you are running; no help will be given if you have mismatched versions!
How do i use them? You simple follow the instructions below, mainly consisting of two or three changes you need to make before you build your panel.
Instructions to install the theme Enola are here Enola 1.2.0(Panel Version) Enola 1.2.1(Panel Version)
Instructions to install the theme Twilight are here Twilight 1.1.3(Panel Version) Twilight 1.2.0(Panel Version) Twilight 1.2.1(Panel Version)
This theme is a recolor of the panel that you can edit, more information here Recolor