Apply only the css in my style.css file

How can I ensure that for a given WordPress HTML block, it uses just the css I provide in my (large) style.css file. I’ve placed the style.css file in a plugin, it looks good, but WordPress insists on lengthening some of the graphics and squishing others.

Everything looks great when I view the HTML file in a browser outside of WordPress, and I’d hate to have to stick !important on every single attribute and add probably quite a few dozen more.

It looks like the following post helped with my problem: