Sometime we need to migrate theme from one installation another. We can do this easily by following the below steps:

1). Create a new theme in Liferay 6.2 using this command.

./create.sh themename ThemeDisplayName

themename = The Name of your theme.

ThemeDisplayName = The Name of theme which you can see in Manage page>look and feel in liferay GUI Application.

Note: Assign execute permission to your "create.sh" in "/liferay-plugins-sdk-6.2.0/themes/" folder.

2). Deploy the created theme by command.

ant deploy

3). Copy the oldertheme(6.1)>docroot>_diffs>images folder and paste it in newtheme(6.2)>docroot>_diffs.

4). Copy the oldertheme(6.1)>docroot>_diffs>templates folder and paste it in

newtheme(6.2)>docroot>_diffs.

5). Copy the oldertheme(6.1)>docroot>diffs>js folder and paste it in

newtheme(6.2)>docroot>diffs>js.

6). Compare olderthemes css files with new themes css files.

Ex. :   Lets take portlet.css. So,first compare oldertheme(6.1)>docroot>diffs >css >portlet.css with newtheme(6.2)>docroot>css>portlet.css .
If you find any changes in same css class then add those changes in custom.css of new theme. Repeat step-6 until you complete to compare all the css files.

Deploy new theme.

Hope this will help you. Enjoy coding :)