how to i edit an apk for my site from another site's apk file ?

hi developers...
i want to edit an apk from another site's apk...
this site's theme like my site...because same theme...
i think apk will work true if i edit true
i used apkstudio-2.0.3b-windows program and i edited smali/com files and manifest file for new add...
and i repack again...
but there is a problem
i install program but not worked....
if i click app it isnt opening...
where am i mistake ?
how can i fix it?