Is there any extensions or themes for that?
This comes up from time to time. 
In the spirit of cross-pollination, here’s a link to a post in a thread on an LO forum on LO and the MS Ribbon. FWIW!
The link to the forum is no longer available.
@tiberiu - the now-defunct LO forum thread is available from the Wayback Machine. I have updated the link in my post.
I am sure that ribbon is possible.
Someone already create ribbon with java for web-based application.
You can see it here: http://pujanggabageur.kodingen.com/isekolah/
I wouldn’t want to head down the Java path given that LO is heading in the opposite direction.
There is currently no ribbon-esque toolkit or extension or project that I know of which will change the current toolbar into a tabbed one (which is what the ribbon is). The related bug to get a ribbon-esque toolbar implemented as an enhancement is fdo#56711.
There was a Google Summer of Code (GSoC) 2013 idea for “allowing new UI experimentation” (under Linux) which does not appear to have been selected as a project, although an idea to improve the toolbars has been selected. There has been the occasional practical and enterprising user initiative to customise the UI, using available options.
EDIT: Note that the ribbon UI in MS Office is patented technology. Alternative office suite implementations will need to find a method of getting around this patent (presumably by being deemed not too ribbon-like). Refer the answer here.
If the developers makes an option for customizing the sidebar,anyone can make own ribbon style menu if they want.The sidebar performances half ribbon style.I like really Libreoffice and i want it to be more popular than ms office.Right now it is really hard to use it for office personal ,especially works at government.
Looks like in version 5.3 there will be - http://www.omgubuntu.co.uk/2016/12/microsoft-office-ribbon-ui-coming-libreoffice
For those who seem to prefer a different interface you could look at the new NOTBOOKBAR interface available to try in LIBREOFFICE 5.3. Its internal development name is Muffin. See Ribbon interface for more information. The team would welcome feedback.