Hi Richardo,
Congratulations on completing the design..
You can give background-color to a parent element that has the full width of the page. Easy fix there.
Just make sure to check issues mentioned in report after submission. Basically saying Using Section Heading elements at least once. (H1-H6 in any title of a page/component). Also using correct semantic landmark regions to separate your html structure. header, main, footer etc.
Try to avoid using fixed values for heights and widths. Research on units that you can use in css, There is a lot of them. "ch" is better for the card's content. Try them, see what works.
This also helps to reduce number of breakpoints needed.
Make your design responsive as much as you can without any breakpoints. Breakpoints are as the named suggests used when design breaks. For most challenges you can use combination of flex and grid. To better understand this, search of "Intrinsic web design" "Designing Intrinsic Layouts by Jen simmons".
This is one of best go to sites to learn the web right away, information fairly accurate and upto date.
https://developer.mozilla.org/en-US/docs/Learn
Keep building stuff.