You can use Visual Studio to edit files including the css files and master page, but you will never see anything approaching the actual design using the Visual Studio designer. The way skins are loaded at runtime is quite different than design time and the VS designer has never been a very good design tool anyway. For skinning you need to edit css (and disable css caching as indicated in our skinning documentation) and refresh the browser to see your changes. You do get intellisense for css when editing css files in Visual Studio.
You may not always get intellisense on mojoPortal controls used in your project. you will get intellisense on some of the controls if they are declared in your web.config and you reference the mojoportal projects and compile it so the dlls get copied to the /bin folder of your custom project. However you may not get intellisense on all the controls especially the ones that are UserControls since the .ascx file doe snot live in your project.
Hope it helps,