Some fundamental information about the Kendo UI widgets' styling is available at:
The article shows how to apply styling to a specific widget type, e.g. all Grids, all Menus, etc. In order to apply styling to a specific widget instance, you need to use the widget's ID or a custom class in the CSS selector, for example:
#MyGridID .k-grid-header .k-header
The k-grid-header class ensures that the custom styles are applied only to Grid header cells and not to other elements inside the Grid, which have the k-header class.
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