It's very simple:

Open skin zip with winrar old version (<4) or 7zip

then:
ui_android/common/ui > variable_def.ui file, open as txt and search:

<row Email bmp="email_button.bmp#3" visible=(1)
<row Sms bmp="sms_button.bmp#3" visible=(1)

Change (1) to (0)

(zero!)

And so for any button.