Right glider in the web pages and dark theme

I have heavy problems on my HD-screen of my DELL XPS laptop to see the glider right, vertical along the screen, to move fast the position in extra long / high web pages (see news paper Le Figaro!) because the white rand around the glider is absolutely to thin and, in the KDE system settings, I don’t find the way how to change the themes (and not know which other theme would preserve as good as possible the beautiful style of Netrunner rolling!). Where are the settings to find? Which alternative theme download (if not pre installed)?

Hi oui,
can you make a screenshot to show what exactly you mean?

excuse me! not along the screen (only if screen = full window ! This was the error…) but along each high window or window sub areal (like in the middle in dolphin from top to bottom as the field is to short for for exemple /usr/share !

I suppose it is the same answer to this (not answered) question at the forum ask ubuntu:

I probably need to try between the divers dark themes Blackbird, Breeze-Dark etc. which one preserve the beautiful look of Netrunner rolling and give me the adapted glider to see it better work more easy?

Probably copy the files into the sub dir “Default” but may I do it manually?