Unable to set sidepane header font weight to normal.

Version information

4.20.2.

OS: Arch Linux.

Current behavior

Cannot modify the header font weight in Thunar's sidepane, both in treeview and shortcutsview via CSS-theming.

I want to turn the header font to a "normal" font weight via CSS in a GTK-theme I am making. Despite numerous attempts and even seeking help on the XFCE forums, where a user suggested posting the issue here, I have not yet been successful in changing the header font weight to "normal" or "not bold".

Picture of current behavior.

For further clarification, here's an image detailing the issue:

Desired outcome

The sidepane header having a regular font weight.

This is a image I edited in GIMP to represent the wanted result: