custom css help

I had previously copy pasted a prewritten css that would allow  filters for the shop page, however, I cannot find this anywhere as it does not show up when I click custom css or code injection. in the filters, there's a "sort" feature, and I want to remove the options "a-z" and "z-a" how would I be able to do this? could I also add css coding at the checkout page? I want to add a box that states "I agree to the terms and conditions" and I don't want checkout to be possible unless this box is clicked. 

does anyone know how to do these?