There’ve been a lot of changes since 2018 when your old version of Anki was released. Even after that page was added to the manual (2021 or earlier, since it was ported from the old site), there have been posts here about issues with GTK themes. This one from 2022 seemed to have more ideas and discussion than application, but you might get more out of it than I did – especially from the developer here: #28 by dae.
I unearthed this from the old forum in 2018 – right around the time your old version came out, so it might not be useful to you now: Anki 2.1 look on Plasma 5 / Anki Archive / Discussion Area - AnkiMobile/AnkiWeb Support .
If you search the Forums, some of the other GTK-related threads might be more meaningful to you than they were to me.
I don’t have any other suggestions for you, but maybe some of the Linux folks will have ideas.