--accent-fill-rest is a CSS custom property (variable) used by Fluent UI components. If you want to change its value globally, you can override it in your CSS, for example:

:root {
  --accent-fill-rest: #ff0000; /* your desired color */
}

If you only want to change it for the banner, scope it to that element:

.my-banner {
  --accent-fill-rest: #ff0000;
}

Then any Fluent UI component inside .my-banner that uses --accent-fill-rest will render with your custom color.

Alternatively, if you don’t care about the variable and just want a hard-coded background, you can bypass it entirely:

.my-banner {
  background-color: red;
}

But overriding the CSS variable is the more “Fluent-UI-friendly” approach.