You can just use the User.setUserField(uid, field, value, callback) function, by importing the module.parent.require('./user') To set the value while creating the account, I suggest you to check the custom registration field (or something like that) plugin and use it, or taking inspiration from it.

See also Where to put custom CSS file.. | NodeBB. Another possibility is a custom css file overriding the others by having the last <link rel="stylesheet" href="..." /> line in the theme source.