Hello! I'm sorry you are having issues with the program. Please try this:
1. With StimuWrite closed, navigate to the StimuWrite user data folder in your computer
Windows: %APPDATA%\Godot\app_userdata\StimuWrite\
macOS: ~/Library/Application Support/Godot/app_userdata/StimuWrite/
Linux: ~/.local/share/godot/app_userdata/StimuWrite/
2. Delete the file for the essential theme pack stimuwrite-essential-theme.pck it will be in the addons/themes/ folder
3. Start StimuWrite and try to install it again
Please let me know if this fixes the issue.