embedded notebooks: implement light & dark mode support

Created by: novoselrok

Embedded web app now reads the theme from URL params, or uses the default system theme. PR also moves notebook end spacing to the SearchNotebookPage.

To force an embedded web app theme, a theme URL parameter has to be appended to the iframe URL. For example: ?theme=light for light mode, ?theme=dark for dark mode. If there isn't a theme parameter present, we will use the system theme.

Test plan

• Create a notebook
• Get embedding url, append ?theme=light and embed it inside an iframe
• Verify that the notebook renders in light mode
• Do the same for dark mode and ?theme=dark
• Verify that notebook renders in default system theme if no theme parameter is present