Look, you either:
a. use BlastChat default CSS
b. use your own template's CSS
c. use your own template's CSS and you add blastchat definitions to overwrite your own
d. you get
BlastChat CSS definition file, store it on your harddisk, adjust definitions and point blastchat to read that file
e. you create separate file form your .css file an you point BlastChat configuration to use that file.