CSS and the \ (backslash)

I am rebuilding a website for a business I have and decided to just use a template instead of build from scratch. I understand CSS but this one thing has me stumped and nowhere on Google have I been able to find the answer for this.

On my page, I have this tag: <div class="3u 12u(mobile)">.
No where in my css do I have 12u or 12u(mobile).
Instead I have this type of codes: .\36 u, .\36 u\24 { style }.

No where online have I found that css uses the \ to represent anything.
I need to get to the 12u(mobile) portion on the css but can't figure out where it is.

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