externalCSS

This property is to attach an optional CSS style sheet to the spell checking window . If this property is not set, the default CSS will resemble MS-Word's Spell Checker using Windows® XP.

There is a sample style sheet included in the Assets folder within JavascriptSpellCheck .  To use this effectively, it is advisable to study the output HTML from the window.

Example:

var oSpell = new JavaScriptSpellCheck();
oSpell.externalCSS  = "assets/sample.css";