Allow sets of styles for pre-styled html

Consider adding another parameter for posting /Html:

theme: light / dark

This will be useful when the client is in an environment, which:
1. want to leverage the std/pre styles;
2. theme aware.

For example, in VSCode extension, we could detect what the user is currently using, dark theme for example, and post:

/Html?preStyled=true&**theme=dark**
