Add a setting to keep theme colours on documents when using override

tests/reference/baseConfig_novelwriter.conf

@@ -1,5 +1,5 @@
 [Meta]
-timestamp = 2025-01-10 02:00:06
+timestamp = 2025-01-11 00:46:32
 
 [Main]
 font = 
@@ -7,6 +7,7 @@ theme = default
 syntax = default_light
 icons = material_rounded_normal
 iconcoltree = theme
+iconcoldocs = False
 localisation = en_GB
 hidevscroll = False
 hidehscroll = False