If you have to use grep to find a CSS class, you're doing it very wrong. Not having any discipline or organizational skills leads to needing grep to find CSS, it is not something inherent to CSS itself.
In my front end, for example, I have registration-page.jsx and registration-page.less, and there is really zero mystery where the styles are for the registration-page. It uses components like password-input.jsx, and guess what? the CSS for that is in password-input.less. There's no grepping, there's no difficulty finding the classes that are specified. Everything is very orderly and easy to find.
Even if your styles aren't colocated you can use sourcemapping alongside established patterns on how to structure out your scss/less. Just inspect the element and click on the filename next to your selector and it opens in your editor.
And what about projects that don’t exist in a paradigm where css can be collocated with templates/components? Legacy projects? Large contributor teams?
In my front end, for example, I have registration-page.jsx and registration-page.less, and there is really zero mystery where the styles are for the registration-page. It uses components like password-input.jsx, and guess what? the CSS for that is in password-input.less. There's no grepping, there's no difficulty finding the classes that are specified. Everything is very orderly and easy to find.