Themes allow you to change the look and feel of your Drupal site. You can use themes contributed by others or create your own to share with the community. Contributed themes are not part of any official release and may not have optimized code/functionality for your purposes.
You can also create a sub-theme of an existing theme. A sub-theme inherits a parent theme's resources. Read more about Sub-theme structure and inheritance.
You can also view a full index of themes listing only their titles.
Impulse is an easily customizable and light weight theme for Drupal 7.x,The main feature is you can edit the basic colors for your website like header,footer,links,sidebar,title and slogan color etc.It has 15 regions so you can customize your website in any form.The highlight of this theme is its navigation it is
Bartik Plus is a sub-theme of Bartik, and it adds Superfish-enabled drop-down menus. Use it if you like Bartik, but need the main menu to offer more than basic tabs. The code is efficient, minimalistic, and can save you hours of searching and fiddling.
A long discussion about Bartik + dropdown menus can be found here.
To use this theme, just enable it and make sure your parent links have "Show as expanded" selected.
A fluid, flexible grid that resets all style elements and prepares a basic layout for your styling. The Elastic theme prepares a baseline presentation for mobile devices and scales nicely to any size resolution.