Hi partner,
First of all, you should login the portal as an administrator and then you will get the portal editor tool on the page.

Then go to the page which includes the button you want to custom and click "Edit" to open editor page and we will be able to inset our custom css or js code into the page.
For example, I want to chagne the color of "Create" button, so I first find the class name of it which controls the color of the button. In this case the class is ".btn-primary", so let's redefine this style in the "Custom CSS" sessions and save it.


Here' the result :)

Also, if you want to change the color of the words, you could use F12 to find the style and test directly in the F12 developer tools.

When your test results meet your requirements, modify the style as described above.
Hope it helps.
Best Regards,
Leo