Is my theme missing something?

Hi,

I’ve downloaded a new theme about a month ago and since then, every time I add a plugin to my Blog, there’s always a/many error(s) that says ? Undefined variable ? (or index).. and every time I have to go in the PHP files and make the variable (specified in the error) global.

This bugs me because when I go in the plugins’ support section, no one ever says that the plugin shoots an error like this.

So I was wondering, is my theme missing something? Certain functions? Why does every variable has to be global? Is this problem common on it’s just my theme?

Thank you for your time.

Justin