First things first
Location of userstyle.css can be found in Tools->Options->General
Notes and settings are stored in: C:\Users\User\.config\joplin-desktop
By default there is no file userstyle.css, so you need to created it.
All you need is to create your own style and restart application.
Hint: Custom style is coped to temp on applicaton startup, so is main joplin style. This means you can see current style used in C:\Users\User\.config\joplin-desktop\tmp (in my case, your actual path may differ, see above).
