Roadmap

Upload

Implement progress bar for upload, get notified by file stream in form-data, use total file size as reference

Make doesThemeExist private
Make exists() private (as of ES2020 methods this is still a proposal), or even better a private getter method that closes over doesThemeExist variable so not even class has access to it when checks if a theme exists in Scroll Viewport
Use ES6 export default for ViewportTheme and normal named export for PLUGIN_NAME
Make async constructor to load config async using loadConfig(), compute doesThemeExist and themeId instead of in create()