You can't apply styles to psuedo-elements in JavaScript.
You can, however, append a tag to the head of your document (or have a placeholding and change its content), which adjusts the styles. (This would work better than loading in another stylesheet, because embedded tags have higher precedence than 'd ones, making sure you don't get cascading problems.
Alternatively, you could use different class names and have them defined with different psuedo-element styles in the original stylesheet.