Hi @mnd , Just want to confirm,
- is the timeline displayed in portal to customers?
- if first question is yes, then you are looking to change the displayed text background color, is that right?
if the answer is YES to both questions then it should be easy CSS update after you identify where:
Here is what i will do:
1- Open the portal and chrome developer tool the CTL+SHFT + C then click on the text that you want to change it background color
2-Find exact element that needed to be styled using (see below example)

3-add Background: any color to the styling and see if that meat your need

4-after finding the right color, click on the bootstrap

5-copy the whole file and know the name of the bootstrap

6-go the portal studio to either update the same file name with the copied css OR create new text file .css and copy the new styling to it and attached it to the portal CSS

I hope this help