Thank you for your answer,
I tried to create the folder and files, following your answer, but then I get the error message:
Telerik.Web.UI.RadTreeView with ID='RadTreeview1' was unable to find embedded skin with name '3DBlue'. Please, make sure that you spelled the skin name correctly, or if you want to use a custom skin, set EnableEmbeddedSkins=false.
Are you sure that I should create the skin folder directly under the App_Themes folder? Shouldn't I include any .skin-file? The master page I'm using utlilizes a theme called 'Bright', so I have a folder named 'Bright' under App_Themes, which contains a Bright.css file and an Images folder for the images used by that theme.
The skin '3DBlue' used by the SiteTreeview control is in my case stored inside that 'Bright' theme folder, where I have a .skin-file for the skin and also a TreeView folder which contains a Styles.css and an img-folder containing the images used by the skin for the TreeView control.
The above configuration seams to work and using your information I changed the styles.css file and now the SiteTreeview starts to behave much better, although I'm still not quite at the goal of making it look the same as the old SiteTreeview control in Sitefinity 3.5 which used the inherit 3DBlue style/skin, I still have some more tweaking to do!
One important change I had to do was to the actual .skin-file:
|<%@ Register TagPrefix="telerik" Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" %>
| Skin="3DBlue" EnableEmbeddedSkins="false"
I had to change the property EnableEmbeddedBaseStylesheet to true instead of false, this change suddenly made the TreeView control behave a bit more what I wanted, when I control it with the styles.css file.
I'm still struggling with getting all the node lines to display properly, especially when there are sub nodes, and also I can't get rid of some padding between the nodes which break up the continous lines between the nodes, but I think I will find a solution for that soon, trying to override the inherit styles using the !important attribute for each style in the styles.css file for the 3DBlue skin.
Thank you for your help and the links to more information!