The Solitude Theme gives no options in Inspector to change mobile nav colors. This is OK if you are using a banner that is non-white but becomes an issue if you have no banner on the page because the default hamburger icon is white (invisible). The background color of the mobile background dropdown also differs from my preferred color scheme. I am not sure how to change these elements.
We should be able to provide some CSS code to change that color. To do that we need to see the code that the Solitude theme generates. I don’t have that theme, and there are probably a number of forum members who could help as well who likely don’t have that particular theme either. Can provide a link to a live page, even a test page, so we can help?