The easiest to do without template overriding, is just to edit HTML of /Extensions/CarRental/Templates/Front/Booking/template.Step1SearchForm.php and CSS at /Extensions/CarRental/Assets/Front/CSS/style.CrimsonRed.css.
The exact location of each CSS class line in CSS file can be seen with Firefox Developer Edition browser and it’s ‘Developer Tools’ with ‘Inspector’ tool.
If this seem hard to do for yourself, please contact our partners for custom work. See here.