I'm now having the same issue as David has described. Previously, I have used the Web AppBuilder before with no issues but now (the second web app I'm putting together) the themes won't show up.
I'm operating on Chrome right now, let me know if there are any resolutions to this issue. In the meantime I will continue trying things to get this to work.
Do you know if your IT department deployed a Windows update to your workstation recently? Just suggesting to look for changes in your environment.
Can you try to create a web app on a different computer or with a different Windows profile?
I just opened it in Microsoft Edge and it worked! Thank God!
Not sure what the issue was but I'm glad it's resolved. I'm going to do the changes I need to in Edge and then move back to Chrome to see if the themes window will now show up.
I'll update as I go.