This code will center each mobile "card" and give them all a width of 90%:
<style>
.mobile_grid_card {
width: 90%;
margin-left: auto;
margin-right: auto;
}
.mobile_grid_card .text_input {
width: 90%;
}
</style>
I noticed that single-line open ends could potentially stretch out of narrow cards, so I added the ".text_input" part to clean that up.
Am I going down the right path? I'm not sure what you need in regards to the label width as there isn't really a labels column in mobile grids.