[GTK3] Fix invalid colour name in treeview

Summary:
Treeview used an invalid colour: `@theme_base_fg_color` instead of `@theme_fg_color`.
BUG: 414391
FIXED-IN: 5.17.4

Test Plan: See that 414391 is fixed.

Reviewers: #breeze, #vdg

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25476