The code which applies backdround slides in Advanced slides doesn’t work with gradient and video backgrounds as explained by reveal js documentation.
For folks to jump in and offer suggestions without a lot of digging on their part, I’d suggest providing a bit more information if you’d like help with this. For example:
- What theme (if any) are you using?
- A link to the version of Advanced Slides you are using.
- Where is the reveal js documentation you mentioned?
- What CSS have you tried so far?