CSS Coding

Redesigning Web site using the twenty twelve theme I wish to apply custom css to alter how the front page looks and to some other pages. I will give you an example of what I want so there is misunderstanding on what is required. Products displayed on site are supplied via datafeedr so would help if you knew who datafeedr is and what they do. I have firebug so know to a point how to find the css so this will help in understanding each other, what I do not have is a complete understanding of CSS. I will do the work, meaning I apply the Custom CSS where it needs to go so it is only the actual coding that is required as we step through the changes required.