Microthemer is just for styling content with CSS. You can use the CSS content property (see the Behaviour group at the end) to add text to an element, but it’s limited to that. You can’t change text.
There is also an option of writing some JavaScript to change the text after the page has loaded. It’s a bit hacky, but if that’s your only option I could provide you with some JS code to add to Microthemer’s JavaScript editor. I would need a link to the page with the text though (and a description of exactly what you want changed).
