You can customize the font color of every Kendo Mobile ButtonGroup by setting color CSS property inline or with CSS selector. For more detailed information on this subject please review the corresponding documentation
Regarding your other question - yes, you could set the whole ButtonGroup width to a percentage.
On a side note, you could customize the appearance of any particular widget in Kendo UI using custom CSS and the widget's specific class
the Telerik team
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI