@Trishyeah CSS stands for Cascading Style Sheet. It's the place where you add code to affect a site's appearance, not the way it "runs." It has no header, footer or other sections. Any component of a site—like the footer—can be "formatted" here: adding code here "overrides" Squarespace's native code.
@bangank36 gave you CSS code to try—entering it into the CSS area of your site (Design>>Custom CSS) may solve your problem by overriding whatever the native Squarespace code is doing. As CSS affects only "syles" (formatting and appearance, essentially), the code already inherently targets the footer.
@bangank36's code, again:
.Footer-inner.clear {
width: unset !important;
height: unset !important;
}
Hope this clarifies.