First of all, I apologize for the delay in responding to your questions.
All application parts should fundamentally be implemented via modelling, which is why the palette includes a rich set of Display and Mobile templates, and our various tutorials are aimed to demonstrate this.
We appreciate that there may be situations, especially with UI, where getting the exact look and feel, may be very difficult to achieve using pure modelling, and do try to provide the information for overriding the default styles - see the various links for CSS and Client Design.
We do understand the need for tools which would make UI design simpler, and to that end have made a few attempts, such as the html import feature you've try to use, but our experience with actual projects shows that as it is currently implemented it is too limited and cannot be used to fulfill real application requirements, and thus, taking into account our resources decided we will not pursue improvements to it, and instead continue to implement UI in our applications using straight modelling followed by CSS tweaking where required.
I want to emphasize that we have not abandoned the idea of developing an alternative import strategy - one thought is to import HTML files as is, and convert them in full to models built of a hierarchy of Tag elements. We would also entertain other ideas for WYSIWYG UI design, but as always 'the devil is in the details', and especially so when resources are limited.
Regarding both problems you reported (css in import html demo & themes), you're welcome to export the projects (File>Export>Tersus>Tersus Project File) and post to the forum, so that we can have a look.