Quick and Dirty way to create a custom theme in Magento

Copy HTML file
1) GO TO app/design/frontend/default
2) Make a Copy of Modern Theme and rename new theme to custom

Copy CSS file
1) Go To Skin/frontend/default
2) Make a copy of Modern folder and rename it custom


Now Go to Admin section 

MENU STRUCTURE Will be System->Designs  (index.php/admin/system_design/index/key/aab859ffebbcf38f6f93e5223088ebcd/)

     Please select Custom design from Drop Down .