Which background are you trying to change, precisely? I’m assuming you mean the background of the image viewer and also the background of the thumbnails in the item/photo grids, but I think those are not black (in neither the light nor dark theme). In any case, you can override the UI styles as explained in this thread here, but could you post a screenshot of the legibility issue? Though transparencies are often difficult to solve in general, we would like to make most images work with the default themes.
There’s currently no way to import Illustrator files directly. If there’s demand for this we can certainly take a look at the format, but Tropy is currently not well suited for vector graphics: when you import SVG or PDF, the images are rasterized at a given resolution and then used. That’s because the image viewer and all effects are currently implemented for bitmap formats. Therefore, importing Illustrator files, if there’s a sensible pathway to rasterize them, is certainly an option. But going forward, it might be more interesting to have an alternate viewer built-in for vector graphics.
If you still have the log file, we can probably compile a list of all the files that failed to import (Tropy creates a new log file when you start it, keeping one copy of the previous log file). Otherwise, if you can compile a directory listing you could compare it to a list of the photos in your project file.
A quick way to get a list of all photo files in your project is to use SQLite, for instance:
$ sqlite3 project.tpy "select distinct path from photos order by path"
Thanks for the encouraging words!