This add-on actually changes the current theme to match the container colour. The idea is great, the execution, not so great. Non-contained tabs, instead of keeping the active installed theme, are pushed to Firefox's built-in primary default theme.

Also, the author reports a bug on Firefox regarding the impossibility of using pastel versions of the container colours. Considering that the add-on is capable of changing themes on-the-fly, instead of relying on Forefox's whatever-buggy-whatever, it could allow the users to select different themes for each container and one for uncontained tabs, effectively solving both problems.

Very useful addon, but the colors are really too flashy...
When do you think you could implement pastel colors? Do you have news for the Firefox bug? Could you give the bugzilla url for this bug?
The best will to give an option for transparency from 0% to 100% (you could add a config slider in the extension settings).

When the colours issue is sorted out this will be a good extension.

When selecting a tab which is not inside any container the theme is reset to the default theme, not the one I have selected. Also maybe try to adjust the colors when the dark theme is selected. They are kind of bright.