The R2 2021 update causes all controls in Classic RenderMode to change their layout and size due to different font-size, font-family and line-height.
Fixing this change is planned for the upcoming Service Pack.
In the meantime, you can use the styles in the attachment. Due to the fact that the selectors are the same as the ones in the built-in skin, you need to load them either in the beginning of the <body> or using RadStylesheetManager. If you are using the StylesheetManager, ensure you set a big OrderIndex for the entry:
<telerik:RadStyleSheetManager runat="server" > <StyleSheets> <telerik:StyleSheetReference Path="fixes.css" OrderIndex="99999" /> </StyleSheets> </telerik:RadStyleSheetManager>
Loading the styles in the <head> tag's markup would not work as the Telerik stylesheets are added programmatically at the end of the <head> tag, meaning it will override the preceding files that have the same selector.
The attachment contains a file for each skin and inside it, it has all styles grouped by control. That means you can load only the file for the skin you are using and optionally remove the styles for controls that you are not using. Keep in mind that some more complex controls(e.g. Grid, Editor, DropDownTree, etc.) contain other Telerik controls as child controls, so you would need to keep the styles for the child controls also.
Please excuse us for the inconvenience caused and thank you for your understanding.