Ramones is right on that. Due to the nature that TEXTBOX is fully a HTML DOM object and not a C2 object. This led to the design that the TextBox and all the DOM objects should be set by the CSS style. Also I hear you can copy/paste a CSS file into index.html to set the value of these.
However as a personal note for game development. If you need text for a game you should be using SpriteFont and no DOM objects. The DOM objects should only be used for more application, form types of input screens.
The reason for this is because DOM object are heavier on the renderer. Desktop computers can get away with this due to the amount of power. But it's still not the better design choice.
If you need some diversity to SpriteFont. You can make a basic white font. Then apply a colour effect that can be modified run time. Also use larger sprites for the font; that way you can decrease the size without loss of resolution that you would get from upscaling.
Finally another reason to NOT use TextBox and the entire DOM family for games. Is that browsers and O/S have effects on the font and font values used. While you can influence some control over this. it is likely that you would need to set numerous CSS settings to accomodate different browsers and OSs as needed.