I did not quite understand from which to which RadControls version you have upgraded, but generally, in Q1 2009 we introduced new skin appearance for all controls and new CSS class names for some controls. If you have some custom skins, you can migrate them with this tool:
Depending on the CSS code in your custom skin, you may need to make additional tweaks after migrating. In some cases it will be even easier to recreate your custom skin, using some of our new embedded skins as a base. Generally, I would recommend you to create custom skins with our Visual Style Builder
and "shift colors"
options, which is fastest and easiest.
Speaking of the style builder, yes, it does not cover every single CSS rule in our embedded skins in the WYSIWYG interface, because some elements are styled with several CSS rules, which are inter-related. You can modify these styles manually in the CSS code.
the Telerik team