My apologies for being so late in replying to this! Here's how to install the theme:
- Download the theme/theme pack/addon to your computer. Do this from the website, do not use the itchio app.
- In StimuWrite version 2 and above, click the "Options" link button at the bottom of the writing panel, next to "About"
- Select "Install Themes & Add-ons"
- Click "Install Theme or Add-on" button and select the theme file in your file folder
- That's it! Your new themes will appear in the Theme dropdown
Let me know if you have any more questions.