Selecting the main CSS sheet

Forgive me but I am new to WP. I am used to Dreamweaver and building from scratch. I am digging through the files to see where the theme selects the correct CSS file. The person who set this up used theme Basic 3.7 and there is many style sheets in there. From trial an error I see its using a style called style-red. There is also style-purple and style-green etc. Where do you select it?