Is there a guide to installing themes?

[Guest couzo]

Hello,

I am trying to instal the black theme but can't for the life of me work out how to do it.

Does anyone know a good guide to do this. I have downloaded the black theme files but when i transfer them across i click on them thinking that would instal them but i just get a message saying that the do not have a program associated with them?

Any help would be great.