There should be a tool to help manage and use themes

some ideas:
```bash
leftwm-themes add https://github.com/lex148/leftwm_theme_bla
leftwm-themes remove leftwm_theme_bla
leftwm-themes update
leftwm-themes update leftwm_theme_bla
leftwm-themes next
leftwm-themes previous
leftwm-themes set leftwm_theme_bla
```