**************************************************************** 2. CSS 样式处理 **************************************************************** 
读取并解析样式表文件
@return {CSSStyleSheet}
@example
CSSStyleSheet: {
cssRules: [
{ // CSSStyleDeclaration
selectorText: "img",
style: {
0: "border",
length: 1,
border: "none"
}
}
]
}
function readStyleSheet(fileName) {

    fileName = path.join(spriteConfig.workspace, fileName);
    if (!fs.existsSync(fileName)) {
        return null;
    }
    var content = fs.readFileSync(fileName);
    var styleSheet = CSSOM.parse(content.toString());
    return styleSheet;
}