Hello I m using LibreOffice and I have several questions:
First of all how to change the background to look like this :
If I could even personalize it I d be so happy
The toolbar as well in blue moon 
Please don’t post an answer if it is not a solution to your initial question. Thank you.
For general look and feel, change the OS desktop theme. Many UI “parts” in LO retrieve the OS theme.
The remaining “LO-private” aspect is controlled from Tools>Options, LibreOffice>Application Colors.
EDIT
It appears that OP requires a bi-color background with a diagonal separation between colors.
Unfortunately, this is not possible. To achieve this, a custom window manager (WM) is needed. LO uses whatever WM is available on the host system. Selecting a WM is done in the OS; LO is fully transparent about it.
If you are a programmer, you can have a try at it. But be prepared for difficulties.
To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!
In case you need clarification, edit your question (not an answer which is reserved for solutions) or comment the relevant answer.
@Astur: ebot is not the comment author. He only transferred Janus49’s “non-answer” into a comment. You should ring Janus49 instead (but this is implicit because he owns the question).
