I bought a custom theme and have the CSS but I cannot figure out how to disable the current theme. It retains the theme even when I check the box at the bottom to only use the new CSS. I am sure this is a common question people have, but I just cannot figure it out. Any help is appreciated. Thanks!
It appears like you are trying to apply the CSS from another theme outside of wordpress to a theme here. You cannot do that. CSS is theme specific and each theme author decides how they are going to organize the markup (XHTML) and the CSS, and what they are going to name all the different selectors (classes and IDs).