@mtrebron is right, we removed the editor. If you need to add or change a theme TTW without having a real Python package, you can still do that. Download the theme, do your changes and upload it again.
Don't know how it could be 'technically done', but having a child theme (similar to Wordpress etc) would be nice (in other words, a theme where you just put the files you want to override). For example the favicon, a CSS file or similar.
it's actually not that hard.
i did something like that for a University.
we have base theme and extending it.
most of the time only by SCSS/CSS but for some sites also the HTML template and/or Diazo rules. if you include images/icons relative, you would have to define theme in every sub-theme. by default you would use an absolute path like ++theme++mybase/...
So to override you have to place the files and change from absolute to relative path.
maybe doable in a project if there is funding.
It could be done similar to how we use the barceloneta resources now via npm.
In my example we where having one theme as you can create it with plonecli and then we created some sub themes next to it. The code is really not much, it basically reuses the rules and theme file from the main theme and provides a scss file which can be customized. If one need a bit more customization for a sub theme, they can also use there own index.html or Diazo rules.
But we managed most of the time just by using SCSS/CSS + logo + favicon.