I've installed a theme from the addon store and want to make a few small changes.
https://color.firefox.com/ has no mechanism to do so, or if it does it has utterly alluded both my clicking every button on the page and reading a dozen forum threads on the matter.
Expected behavior:
A button next to 'export', maybe labelled 'import', which allows the user to select an installed theme or upload an xpi file.
Actual behavior:
Nothing at all.