Pretend your WordPress theme is a house. The theme’s HTML is the foundation. It contains elements such as paragraphs, headers, images, and divs. It’s the structure of a document.
The theme’s CSS is the interior design. It describes the appearance of the HTML elements: their colors, font faces, spacing, and backgrounds.
You can keep an HTML document the same and only change the CSS. It’s like redecorating your home, or putting on a new outfit. Or giving a car a new paint job.