Smooth the theme switch

[pandada8]

• fade out the page before switch page
• after the new css loaded and page reflow finished, fade in the page

could intergrade with #62

[jamesliu96]

maybe this procedure is better:

• start fetching new css
• after the new css loaded and page reflow finished, fade out the page before switch page
• fade in the page after a short time interval

this may shorten the "empty view time" of the page, and while fetching the new css, show a loading sign.