Themeroller is the online WYSIWYG jQuery UI editor. It is pretty much straightforward to use and that makes it really popular.
jqGriid offers full Themeroller support, i.e. any Themeroller produced theme applies to jqGrid automatically when placed on the same page. Rather unfortunately, Themeroller itself does not offer any jqGrid support and more often than not it is hard to figure out what parts of the .zip file Themeroller finally produces apply to jqGrid and how to use these.
Fortunately, it turns out to be pretty simple. The only two items one needs in the jquery-ui-<version>.custom.zip Themeroller produces are the ./css/start/jquery-ui-<version>.custom.min.css file and the ./css/start/images folder. These should be extracted and published on the web. The important part here is that the 'images' folder should be a subfolder of the folder where jquery-ui-<version>.custom.min.css is located. I.e. the 'images' folder and the jquery-ui-<version>.custom.min.css file should preserve their location relative to one another. This is important because the jquery-ui-<version>.custom.min.css uses relative paths of the kind './images/someimage.gif'.
Finally, one should add a link to the jquery-ui-<version>.custom.min.css in the <head> section of the webpage that contains jqGrid. Typically it looks like this:
<link rel="stylesheet" href="/jqgrid/mydomain.com/mycssfiles/jquery-ui-version.custom.min.css" type="text/css" />
And as mentioned above the 'images' folder extracted from the Tehemeroller produced .zip file should also be copied to mydomain.com/mycssfiles/.
That's all you need to make jqGrid use the Themeroller produced UI.