It should work if you add
Alpha: 255; to the [mousefocused] state, because DirectUI elements inherit properties from its previous state. For example, if you had an element with normal, mousefocused and pressed states, and you only specified the image file in the normal state, it would stay that way, but if you specified different images for each, the element would visibly change. This applies to all properties and states, as far as I know. The WLM team don't exactly follow this much, it'd probably save a heap of space, but oh well.
Hope this helps
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