The easiest way to install these themes is using my Theme Customiser tool. Alternatively, you can find install guides for some themes on the wiki.
For further help, I have various forum threads on the MyAnimeList forums where you can find a history of previous user questions and solutions: threads.
You can report issues either of two ways:
If you have design feedback about Clarity, 9anime, or Tilt, you should know these themes have been essentially feature-frozen. Due to how I had users install them with auto-update functionality, I will not release anything beyond simple bug fixes or support for new features.
If you have feedback on any other theme, feel free to contact me or open an issue and I will take it into consideration.
Feel free to modify and use any of these themes however you wish to. My one request is that you do not claim credit for the original themes. I do not mind if you take snippets of code to create something new. Code being reused is commonplace; What matters is the visual result is sufficiently different.