Themes is a great way to customize look and feel of your application and an easy one too. You can customize them or design your own but designing your own theme requires a lot of expertise and time. From my point of view its not a great idea to invest in designing your own theme provided you have so many themes available for you. You can take one which suits you and modify it as per your needs.
Xmarks is one of my favorite add-in. I have been using it for more then two years and it made my life so much easier. Had no tension of keeping my bookmarks while I changed my job, formatted my system or installing new browser. It was so easy to keep my bookmarks intact across browsers and PCs that I am still wondering about how am going to keep my bookmarks after Jan 1’st.
I still remember my frustration with various bookmark solution I had tried before finding Xmarks. The best thing about Xmarks was how user friendly its website is, exactly what a Bookmarkholic like me wants.
I hope they find a buyer who can appreciate what they have done.
Best of luck guys and thanks a lot for making my life simpler for all these years.