You can do this:
Pretty convenient. Zipping is not needed unless you want to send/share the theme.
I tried my best in trying to reproduce your problem but was unable to. I followed the steps you took but had no problem with the theme itself. Perhaps you can clarify your steps a bit more in detail.
With regard to the title of you app: the value that is entered in your theme's index.html file is overwritten by the official app name as present in the modeller. You can adjust this name in the project settings in your modeller.
The fact that you managed to get a white screen sounds like the theme itself is either not properly in your theme folder, the project settings don't link to the correct theme or that an error has occurred with zipping your theme.
What you could try is deleting everything in your theme folder (maybe make a backup first of the folder) except the zip file. Extract it on the spot and put your project settings theme to default. cleaning your deployment folder is always a good test to do as well, after which you should run it again. If the problem is still there, than the index.html file is probably no longer correct or somehow the css file is no longer proper.
With regard to your generic need: we're always working on making it as easy as possible to style your app in Mendix. This will however encompass big platform changes so it's not going to happen soon. If you want to learn more about styling in Mendix we have a styling workshop which deals with getting the proper knowledge and workflow for styling in Mendix. We will also be expanding our knowledge sharing in the near future with regards to that aspect as well. For now, for styling I can give you the advice of stop working with zip files: use the default option which uses the base theme folder as input. And perhaps with major releases you could zip your theme and use that as a milestone. Other than that, short term advice would be to look into the styling workshop of Mendix.