What to add in MS thesis presentation
Your advisor can give you the best advice. You might also attend another student's presentation to get an idea of what is typical at your institution. 

But probably you should cover the ground lightly but be ready to supply more detail as requested. Focus more on your own contribution, but don't neglect the basis on which your work rests. Probably a 70/30 or even 80/20 split seems about right. 

If you haven't given enough detail, I assume you will be asked questions, so be prepared for that with a more complete explanation.