How do I download it from github?
You can download it from here: https://github.com/covedesign/openrov-simple-theme/
There is a "Dowload ZIP" link on the right hand side.
Now I don't understand how to install it.
Where and how shall I put the folder?
To install it, I first connected the OpenROV to my home network via ethernet cable plugged into top side adapter. On boot OpenROV gets a DHCP IP address (you can ask your DHCP server which address it got).
Then I unzip'd the plugin on my local machine, then used "scp -r" (secure shell copy program) to recursively copy all files to /opt/openrov/cockpit/src/plugins/...
You can also use file sharing: http://openrov.dozuki.com/Guide/How+to+create+a+software+plugin+for+OpenROV+Cockpit/22 (scroll to very bottom) ... I haven't tried this way yet ...
what is "scp-r"?
Im quite a noob on programming and stuff....I like plug n play ;)
"scp" is just a program for copying files from one computer to another, over secure shell. The "-r" flag just means "recursively copy all contained files/sub-directories as well".
Probably it's easier if you set up file sharing (from that guide)? Then you can drag & drop the plugin files onto your OpenROV.