Using Eclipse Photon and latest DevStyle as of today.
When Darkest Dark theme is enabled and you switch editors no matter the method (click on tab, CTRL+PGUP/PGDN, CTRL+F6, …), you can see the native vertical scrollbar flashing briefly before being replaced by the themed scrollbar. It also happens on the horizontal scrollbar in some cases. This is a bit distracting and the stock Eclipse dark theme does not have this issue.