How do I modify the color for the heading in the new UI Builder page when using Portal App Shell

[Anil V]

I create a portal using the new UI builder and am trying to figure out how I can modify the color for the heading. 

My setup is as below

1. Created a new Experience with Portal App Shell and created a UX App Config for my new experience.

2. Created a theme and linked it to the new experience.

3. Created a page, with a component on it, I used Richtext component for my testing.

in the theme I added below, and was expecting bluish background for the header, but it didn't change color.

theme:

{
"--now-color_brand--primary" : "5,49,77"
}

Any Help is appreciated.

[Brad Tilton]

I tried Michael's suggestion and it worked for me in changing the color of the header background in the portal app shell.