I use the dark theme, but its hard to see the scroll bar - ss http://i.imgur.com/hp2cnEB.png the scroll bar becomes lighter on mouse over, but its hard to see otherwise, especially if the bar is small due to lots of text in the note. The bar I believe is too similar in color to the background color so its hard to see.
This is especially true if I am using my computer in well lit room - which I normally am!
ps. Joplin is awesome