Trello related CSS was dropped in Wekan v0.9 2015-09-10, so after that Wekan does not use anything from Trello. UI and feature set is different.
https://github.com/wekan/wekan/wiki/FAQ#why-does-wekan-look-...
https://github.com/wekan/wekan/blob/devel/CHANGELOG.md#v09-2...
According to Open Hub, Wekan currently has about 9000 lines of code without counting Meteor.js framework and related packages.
https://www.openhub.net/p/wekan
I don't remember did Wekan original creator Maxime Quandalle mention why did he choose Meteor.js in his talk at YouTube:
https://www.youtube.com/watch?v=N3iMLwCNOro