How do I customize the Gridmarket search bar in the side bar widget?

Hello. I have a GridMarket multisite coming along. I would like to refine the search function in the side bar widget by having:

1) search form on one line; width 100%
2) search submit button below it; width 100%, and height reduced (making a thinner button).

I tried to start by adding css values to GridMarket with no luck.

Thank you.