I don’t use a theme that changes the pdfViewer so i dont know for sure, but you could try to just replace pdf-viewer to pdfViewer in the theme.css file. It should be located at Obsidian Vault/.obsidian/themes.
Maybe they also changed more variables that breaks other stuff in your theme, so if that fails, I would advise you to contact the developer of your theme and ask them to update it.