overriding a style in android.

Question

Let's say i have a layout with style:

<LinearLAyout ... >
    <TextView ... style=someStyle />
    <ImageView ... style=someImageStyle ... />
</LinearLayout>

the style will be defined in an xml in my project. How can i override that style with an external xml ? (i'm asking because i've noticed the View does not have applyStyle\setStyle or anything of that sort (best bet, because style need to be parsed, compared against android:attr for validation and then applied on each item of the view). I do wonder how am i suppose to make downlaodable themes for my app.