Rewritten from my original article, in the writing style I use for Discourse.
Introduction
Oh come on, you're writing useless posts for likes now!
Hold your colour until you understand what this is about. KDE has some beautiful cursors — These colourful, slick-looking inverted pen nibs with smoothly-animated meditation balls are well-renowned by users inside and outside the KDE community, but there's a long-standing issue with them.
For Debian-based systems, Their file names, while accounting for most circumstances fail to do so for the window borders like the old DMZ set, or the current MATE set does, so resizing a window shows ugly default cursors instead of the awesome cursors users who install oxygen-cursor-theme and oxygen-cursor-theme-extra expect. With some effort on a user's part, oxygen themes in use can display what they're suppose to.
Fixing the arrows
To fix these problems and ensure the appropriate graphics appear when resizing windows, change directory into the oxygen theme set you want at /usr/share/icons and perform the following as a superuser in its cursors subdirectory;
ln -s size_fdiag top_left_corner
ln -s size_fdiag bottom_right_corner
ln -s size_bdiag top_right_corner
ln -s size_bdiag bottom_left_corner
ln -s size_ver top_side
ln -s size_ver bottom_side
ln -s size_hor left_side
ln -s size_hor right_side
You can even save this as a shell script, and link from that into the oxygen cursor themes you want to fix. Don't forget to chmod +x it, else you may not be able to purpose the above in a shell script.
Corrections
Forgot to link in my original article, as crappy and awfully-written as it was. The information is still there, though, same as it ever was. (Bonus if you get the reference.)
Also added advice for chmod +x because I figured it wouldn't hurt to include.