Every blog here on this multiuser blogging platform using the same theme is using the same underlying template and we cannot edit themes or templates on free hosted WordPress.com blogs. Also CSS editing cannot be used to change functionality. It can only be used to change appearance. Whether or not a linked title to posts, a full post, or thumbnail images are displayed on Categories and Archives pages is theme dependent. See here for a list.
Some (very few) do have active Excerpt boxes. They can be used to copy as much content from each post from the HTML editor as desired and paste it in the Excerpt box. The Fruit Shake theme does not have an active excerpt box so this is not available to you. In other words, what you see now ie. full posts is what you get, and cannot be changed.