Fix invalid color map for treeview

This resolves an issue where the a Tcl app with the dark theme applied would throw an error when selecting row in a treeview.

This doesn't affect the sv-ttk Python package as this bug was only present in main.

sv_ttk/theme/dark.tcl

@@ -438,7 +438,9 @@ namespace eval ttk::theme::sv_dark {
  -rowheight [expr {[font metrics SunValleyBodyFont -linespace] + 3}] \
  -font SunValleyBodyFont
 
- ttk::style map Treeview -background {selected "#292929"} -foreground {selected $colors(-selfg)}
+ ttk::style map Treeview \
+ -background {selected "#292929"} \
+ -foreground [list selected $theme_colors(-selfg)]
 
  ttk::style element create Treeview.field image $I(card) -border 5 -width 0 -height 0