Activating non Upfront theme and modifying "front page displays" strips styling of Upfront

I'm trying to understand why my Upfront Builder theme "Vanell Arts" (built from Scribe template) has no styling after I activated the "twenty fifteen" generic theme. I had also changed the "Front Page" in Settings > Reading to a static page I had created in "twenty fifteen". I had done this because I was not ready to launch my Upfront theme. When I went back to edit my "Vanell Arts" theme, I got a stripped down version. Colors and fonts have been removed.

Not sure what happened here.

Additionally. What is the best practice for when I want to have a "Site Under Construction" landing page while I build the main site? How would I accomplish this in Upfront Builder?