Hello! I'm sorry you are having issues with the program crashing. Please try this:
1. Navigate to the StimuWrite user data folder in your computer ( you may have to unhide hidden files and folders)
Windows: %APPDATA%\Godot\app_userdata\StimuWrite\
macOS: ~/Library/Application Support/Godot/app_userdata/StimuWrite/
Linux: ~/.local/share/godot/app_userdata/StimuWrite/
2. Delete the addons folder
3. Delete stimuwrite.cfg
If you like you can send me your godot project file for the custom theme and I'm happy to look at it to see what the issue was, my email is stimuwrite@eveharms.com . Feel free to email me if you have any other issues also.
As for custom video backgrounds, this will probably not be a feature added any time soon due to the way it was programmed and how the engine handles video. Unless I rewrite it, adding a system for users to import their own video backgrounds would be almost as complicated for them as creating a custom theme from the template. And even if I did rewrite it, it would still be too complicated for most users because of the engine. But I'm happy to help you with your custom theme.