If you want to override Android default themes, first make sure you can target the correct platform - the themes have changed.
Second, you can override one property this way:
Override default Android themes
Third, if you just wanted it done for an activity, you can apply the theme for the activity in the manifest.
Or, if you are just looking to override the color only, you can always add the line:
android:textColor="@color/theme_color"
to each textview you want to appear that way (it will override the style)