I am not entirely sure how things will behave if we disable the OS theme style from the the list/tree views; as this might introduce some unexpected rendering effects.
This requires FlashFXP v5.1.0 (build 3848) or newer.
Download the latest FlashFXP release
1. Start FlashFXP
2. From the main menu > Commands > Raw Command
3. Enter the following command
/set app.os.theme 0
The theme style should now be turned off for the listview and treeview.
Please let me know if this setting has any unexpected side effects.