If you need a search form in your header, you can add it with the “Search” grid-part.
Using the “Padding” option you can change the left and right padding of your searchform.
You cannot change the vertical padding as the form always takes up 100% of the space assigned to this grid-part and is vertically centered.
Using the font-size control you can change the size of the text inside your form. Please note that the sizes selected in this field are relevant to the body font-size, so a size of 1em will be the same size as your main content.
Finally, using the background-color and tet-color controls you can change the colors for this grid-part.