Go to your project directory > ‘\theme\styles\web\sass\app’
Create a new folder for your widget or UI resource (in your case this will be ‘Widgets’)
Add a new file under your new folder (in your case filename will be ‘_navigationbar.scss’). You can have a look at available widget filenames by navigating to project directory > theme > styles > web > sass > core > widget.
Open _custom.scss from app folder and import newly added file, (in your case, add @import "Widgets/navigationbar" and save it
Go to newly created file and add required SCSS code and save it so that Calypso compiles the code. (in your case go to _navigationbar.scss add below suggested code and save it)