One of the ugliest things about Thunar, besides the fact that it can't keep a layout, is its navigation arrows.
This problem gets even worse if you want a good looking system and use more than one file manager.
File manager navigation arrows with different themes:
This can and should be fixed, because it is easy to fix.
All you have to do is find a few icons you want to use and rename them as shown in the lower part of the screenshot.
That was it.