Get the files:
mkdir -p ~/.local/share/fcitx5/themes
cd ~/.local/share/fcitx5/themes
git clone https://github.com/lilydjwg/fcitx5-skin-violet
Then choose the "Violet" theme via fcitx5 config tools. Tutorial from felixonmars: https://twitter.com/felixonmars/status/1344335122787573760.