I’ve uploaded a few themes with some issues with the colour of tab titles and I’d like to fix that, but I can’t see an edit button on the theme page even when logged in. Is it no longer possible to edit old themes or am I missing something? I could upload new ones but they’d probably be rightly deleted as duplicates.
It is still possible, just not quite as easy as before.
If you go to your designer page, you should see a Tools link at the top of the page. From that menu you will find a Manage My Submissions. On that page is a Themes tab, click on that. And since the the themes are set to show them by Name as default, you might want to click on the Created link and it will show you the most recently submitted themes.
However, if you want to be able to do it the way you used to, you can do so by going to the add-on homepage, scroll to the bottom of the page and click on View classic desktop site which will let you use the old site design.