I know I sound like a broken record when I praise Windows 95 UI, but holy fuck Microsoft figured this shit out already about 30 years ago. It's all there, black and white, clear as christmas:
So much of modern UX woes stem from not knowing, or intentionally ignoring the genuine design study put forth into GUIs in the 90s.
3D elements are 3D in a specific way with lighting from a specific side to make it obvious where a window element begins and ends.
The gradient always should from from one side, and keep it consistent.
Make your color shading and shape of scroll bars consistently side and easy to press. I have a 4K display, don't make me hunt for the magic activation pixel that makes your 3-pixel wide scroll bar appear.
It's a desktop application, I've got the screen real estate to spare to have the actual GUI elements present on screen at all times (I know, heresy).
The moment aesthetic takes precedence over form and function, you've failed as a UI designer.