Highlight treeview header button border with more contrasting color on hover

Issue:

The existing treeview header button border is close to invisible. The preferred way to find the border for a treeview column was to hover the mouse near the beginning of the header button text expecting a switch in mouse pointer to resize mode. This worked when all column headers in the view are left aligned. After adding a few numeric columns to the view, which were right aligned, it became really hard to find the location of the border between a left aligned and right aligned column header.

Fix:

This change addresses the issue by highlighting the right border of the header button with @borders color which has more contrast than the current one, when hovered. The original intention was to highlight both left and right borders, but since treeview header buttons do not have a left border ( 0px ), the change settles for the right border ( 1px ).