Write CSS into file with Javascript

I am working with a corporate system (blackboard LMS) that does not allow me to have access to the serverside code and or the HTML/CSS. I need a Javascript solution to enable additional css rules to placed into the document, so i can override page styles. I am able to embed JS and CSS styles into a page. I need a solution that allows javascript to do ONE of the following (whichever works the best) a) Put a new link to a new external stylesheet reference in the head of the page .. or b) Take an external css file and write the css into a style tag on the page I cannot use jquery (there are conficts/issues beyond my control) but Blackboard does use prototype so if it is not possible without any sort of library, you could piggyback off of prototype. It needs to work in ie8+, FF, Chrome, Safari I cannot give any access to the system, I just need a standalone solution that I will implement. Thanks Dean