make the scrollbar thumb high contrast
Right now scroll bars have a light gray bar with a gray thumb. There needs to be way more contrast between the scroll bar and the thumb. Terminal windows usually have a very dark of very light background and when staring at that for a while, it takes a long time for eyes to adjust to distinguish between the low contrast scroll bar.
I am not sure if this is an XFCE issue, a theme issue, or a terminal issue, but it seems most critical for me with the terminal since scrollback buffers can get very large and then that makes the thumb very small. I have not been able to figure out how to change this with themes though.