change theme with to 75%

hello! My theme is extremely large and I’m trying to reduce the size to look as it does when the page zoom is set to 75% instead of 100%. Would anyone know how to do this?

p.s.-I’ve read some other posts related to this subject but nothing has worked. I might also want to add that I am slightly computer illiterate so if you could walk me through through the process step by step that would be a big help. Thank you in advance!