The usage of the most common Kendo UI CSS classes is outlined at:
Depending on the exact context and theme, a given CSS class may apply different styles. This means that a given CSS class may or may not produce a visible effect when used outside a Kendo UI widget.
Specifically .k-content applies border-color, background-color and color styles, depending on the theme.
.k-group applies border-color and background-color styles, and box-shadow when inside a Menu widget.
Both classes apply border-style, border-width, width, height, margin, padding, position etc, depending on the context (widget).
Note that an applied border color style does not automatically produce a border, unless a border-style is also defined. This is defined for cases when a Kendo UI CSS class is used inside a Kendo UI widget, but not necessarily when used outside a Kendo UI widget.
Theme-related styles are located in the theme CSS file, while context-related styles are located in the common CSS file.
For more information, I recommend looking inside the Kendo UI CSS files and / or inspecting the HTML markup with Firebug or a similar tool.
Let me know if you need specific additional details.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI