From KDE UserBase Wiki

    Backgrounds and Edges

    The background is intended to approximate the look of high quality paper (e.g. card stock). For Plasma Workspace elements, the background is (barely) translucent. For applications the background is fully opaque. The general background color is Cardboard Grey.

    For more information-dense content areas (whether in plasmoids or applications) Paper White can be used for improved readability. These might include longer lists, tables, large blocks of text documents, charts,etc.

    Edges are sharp and distinct. Corners are sharp but non-threatening; about a 2 or 3 pixel corner radius.