GTK+ in Kubuntu apps look bad
Make sure you have the following packages installed (they should have been installed by kubuntu-desktop, but may have gotten missed):
sudo apt-get install gtk2-engines-oxygen kde-config-gtk
Then go to the KDE Control centre and make sure that the checkbox for using KDE themes in GTK apps is checked.
Evolution probably uses GTK3 while the instructions are valid only for GTK2.
I'm not sure what is the policy of AskUbuntu on external links, but this one provides a solution for GTK3:
I accidentally found the right answer here
Old question with old answers, but maybe still interesting - and appearing in internet searches anyway. So, for the record:
In KDE/Plasma5, there are GTK settings in the System Settings, Application Style: