To do this you need to use the custom section on the branding page. You need to inspect the element through your browser and add the CSS class to the custom title.
1) Inspect element like below
<Please see attached file for image>

Step 2) Notice under "Styles" you can see the class that is needed for modification. To test this you can change the background color temporarily through the UI. Just to confirm that you are using the right class.
<Please see attached file for image>

Step 3) Log into the Admin UI for Identity Portal and select the Branding tab.
Step 4) Be sure to select the appropriate View on the top left. If the wrong view is selected then you won't be able to update the color accordingly. For the specific example above I'm using a public view.
<Please see attached file for image>

Step 5) Take the CSS Class specified on Step 2. For this scenario it is ".sigma-button.success" and add it to your Custom section.
Step 6) Select add attribute on the next column and choose background-color and hit Save on the top right.
<Please see attached file for image>

If necessary refresh the page where the change was made and see if the color updated. You should notice it is complete.