@linzo this is a CSP issue, it has nothing to do with the theme component. Please read about Discourse CSP here: Mitigate XSS Attacks with Content Security Policy
@Zup, this is an additional feature request - possible of course but it is additional work which will have to be paid if you want it implemented. Let me know if you want that.
Sorry for the delayed response - yes there is a transcoding process after the upload, and the upload dialog will show that before it embeds the video. This is working as expected on my side. It waits till the transcoding completes, only then it closes the dialog and embeds the video. Here is a demo: