WordPress also have many- many more theme . You can just choose any theme from WordPress theme directories and upload it to on your WP-theme folder . But some blogger/developer wants to see his theme different by other blogger which use this same theme .Like If any developer/blogger select twenty twelve theme but their default color is white and bg color is gray and it’s took some common function .But any blogger who wants to make our theme different to other in that case.
You can do this by using WordPress appearance sub-option editor . Where you can just changed your theme functions or theme style according to your idea without any problem .
step 1: Open your WP Dashboard and choose Appearance option which are in left sidebar .
step 2: Select the editor sub-option of appearance .
step 3: Select theme file and now append your code with theme directory.