The lightbox has several new buttons with new icons, but no text labels. This means that your visitors will have to figure out what the icons mean before they'll be able to use your lightbox properly. This is particularly annoying for the "download" button (which appears if you have the display of Original size photos enabled in your galleries).
You can make the function of these buttons more obvious by adding labels to them. Choose the buttons you want to label from this code, and add the corresponding block of code to your theme's CSS:
Here's an example of how the new labels look in the gallery, and in the lightbox view: