Hi Mik, unfortunately that is not the intended use of theme manager. It is for customizing a base Bootstrap file. It's not going to help you with third party templates. Actually, Wappler itself, is not intended for use with third party templates. Usually easier to. recreate them from scratch than to edit them.
This doesn't mean what you are trying to do is impossible. It will just take more work and CSS/Bootstrap knowledge. Here is a link to get you started on using custom bootstrap set ups.
Seeing as how the theme you bought is based on bootstrapvue this mat not even help you and you may have to contact the provider for assistance on what files you need to edit and how. But there will not be a 'Wappler way' to do this.