Roundcube 1.2.4 has been released. It's a bug fix release, but does fix "a recently reported security XSS issue with CSS styles inside an SVG tag." These traditionally work fine by just renaming the last ebuild. Changelog: https://github.com/roundcube/roundcubemail/wiki/Changelog#release-124
Can confirm: rename of existing ebuild works fine for me.
Now in repository, https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=a83a957baa3f709c0fba7e08af6d195d871f30ff