Download a zipped theme file and unpack to 'lintalist\themes' - some themes may include
alternative icons in the icons
sub-folder. Unpack while preserving the folder structure.
More information about themes and how to create your own:
https://github.com/lintalist/lintalist/blob/master/themes/Themes.md
If you have a theme, feel free to open an Issue or make a Pull request.