search: Fix expand triangle color with light themes

If you are using a non-breeze theme such as fusion and the only color scheme available to you is "light" and you like using dark editor theme, then while searching the expand triangle is almost invisible in search results. This change fixes that.

To test, open kate with fusion style and a dark theme such as "monokai" and search for something

Before

After

Subclassing was necessary as we need to only change the color in the view and not other places of the treeview such as scrollbars.