How to increase font size of preview options when using an input method?

I am using IBus for pinyin in Ubuntu 18.04 (Gnome), with the "intelligent pinyin" input method.

The Chinese characters in the preview box are way too small and barely legible, i.e.:

What I tried:

~$ ibus-setup
~$ # I ticked option "Use custom font" and set very large font size
~$ ibus restart                        # (rebooting the machine also has no effect)

This did not change the font of the characters in the preview box.

NOTE (not a duplicate): the troubleshooting I followed is similar to the one proposed here for a previous version of Ubuntu (11.10). The approach described in the previous Q/A does not seem to be valid any longer, so I made a new question because this issue deserves some renewed attention. It is perhaps worth mentioning that the same troubleshooting was working on the most recent Linux Mint+Cinammon release, so the issue seems to be related with Ubuntu+Gnome.