How to properly style the second header for other pages?

Question

Sass

I am doing a BAM project and dividing the blocks into different files (pug, scss). In the project, on the main page, the header has a dark background and white icons/texts, and on the other pages, on the contrary, there is a white background, black icons and text.

What is the correct way to implement this at the project structure level? A lot of questions are generated at the stage of thoughts, so just tell me how do you do it? where do you write or redefine the styles of the second header, how do you add it to the page? Any advice, thanks.