Do you know if the issue is present in e.g. 20.04 or 20.10 as well?
Assuming you are using Kubuntu, I don't have access to any Kubuntu install ATM. But both Mozc Setup and About Mozc look good for me on Ubuntu 20.10 and 21.04, i.e. it honors the theme in effect, in my case Yaru which is a GTK theme.
Thanks for your report.
Do you know if the issue is present in e.g. 20.04 or 20.10 as well?
Assuming you are using Kubuntu, I don't have access to any Kubuntu install ATM. But both Mozc Setup and About Mozc look good for me on Ubuntu 20.10 and 21.04, i.e. it honors the theme in effect, in my case Yaru which is a GTK theme.