the greenbarring is handled by some css in jquery-ui.theme.css circa line 482;
notice the use of css variables here;
All the css variables are defined and set in nt-theme.css
In the base theme they are declared circa line 580;
so yes, the easier way is just to adjust your nt-theme.css file.
You can put direct colors in here if you like, or you could use one of the defined palette colors (which you'll see declared near the top of the file, and which you can also inspect in the themer example.)
If you like I can walk through this with you in the User Group webinar.