Unfortunately this is a WP8 IE10 browser bug - the text caret is always black, regardless of the input background (on desktop it depends on the background). Also the caret color can't be changed, there's no CSS property or even standard about it yet. As a workaround you can make the inputs a little bit lighter, so that the caret becomes visible. something like this:
Adjust the color and input type. The light theme should be fine.
the Telerik team
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