Thank you for trying the Darkest Dark theme. Glad that you love it. The scrollbar behavior in the editors comes from Eclipse itself. I have filed a bug for the dev team to investigate further.
As a workaround, you can replace the themed scrollbar with native one by adding
-Dswt.enable.themedScrollBar=false
flag to the eclipse.ini (after -vmargs) located in the Eclipse install directory.
Hope this helps. Please let us know if you see any issues.
–Swapna
MyEclipse Support