
This is a function of the QT canvas so you need to add a stylesheet.qss (QtStyleSheet) file to the settings directory see where-are-the-settings-stored
For the green scrollbars I simply copied the example sections
QScrollBar:horizontal { border: 2px solid grey; background: #32CC99; height: 15px; margin: 0px 20px 0 20px; }
QScrollBar::handle:horizontal { background: white; min-width: 20px; }
QScrollBar::add-line:horizontal { border: 2px solid grey; background: #32CC99; width: 20px; subcontrol-position: right; subcontrol-origin: margin; }
QScrollBar::sub-line:horizontal { border: 2px solid grey; background: #32CC99; width: 20px; subcontrol-position: left; subcontrol-origin: margin; }
QScrollBar:vertical { border: 2px solid grey; background: #32CC99; width: 15px; margin: 22px 0 22px 0; }
QScrollBar::handle:vertical { background: white; min-height: 20px; }
QScrollBar::add-line:vertical { border: 2px solid grey; background: #32CC99; height: 20px; subcontrol-position: bottom; subcontrol-origin: margin; }
QScrollBar::sub-line:vertical { border: 2px solid grey; background: #32CC99; height: 20px; subcontrol-position: top; subcontrol-origin: margin; }
QScrollBar::up-arrow:vertical, QScrollBar::down-arrow:vertical { border: 2px solid grey; width: 3px; height: 3px; background: white; }
QScrollBar::add-page:vertical, QScrollBar::sub-page:vertical { background: none; }
From this page customizing-qscrollbar
Of course there are others available but these are the core ones for your experimental mods. To start slowly, change one of the #rrggbb e.g. #32CC99 (grenish blue) to say #222222 and observe the effect. Each rgb HEX color ranges from dark 00 to bright FF for possible values there are several theme or color wheel pickers see W3schools Hex Calculator
[Edit] from Markus's own observations. (some improvements above)
1) You will needs to restart TeXstudio to load/reload the .qss file during modifications.
2) The important lines for his question were lines 5 and 6 of the above example.