Default [Q] Add icons to MIUI v5 default theme

I believe MIUI's default theme icons are stored somewhere. I don't want to set other themes because according to MIUI's new rule, themes downloaded offline (.mtz files) can't be set offline anymore. I just want to add my icons to default theme and use it. Where can I find default theme icons directory (or some packed file where I add my icons and repack)? Or is that the only way to modify some framework apks like framework-miui-res.apk?