Change the header color

Hi, the automatic email that is sent to the people immediately after the subrsciption to the newsletter, asking to do a click to confirm the subscription, is showing a html gray color and I would like to change it into light blue and chang the colore of the title from white to black. how to do that?
Here the picture to udnerstand better: https://ibb.co/rQwNWgp

Are there other automatica messages using that gray color? because I want to use the same company style and not that gray if possible. thanks!

The page I need help with: [log in to see the link]