Gateway – Site Title font white at all times

I am using the Gateway theme, and I am having trouble finding a way to get the Site Title to stay in white fonts on the header image for this site:

https://ourhouseinprovenceno.wordpress.com

Since I want the background color to be dark, it seems like the color of the Site title also changes to a dark color.
How do I change this so that the color of the site title is always white ?

I know it is possible without any ‘coding’, since I achieved this on my other site by default. I just do not know what I did differently:

https://ourhouseinprovence.wordpress.com // this site is OK with white Site title

Any input is highly appreciated!