First of all you may need to unhide the hiden files so open terminal and run those commands :
defaults write com.apple.finder AppleShowAllFiles TRUE
To switch back, do the same but substitute FALSE for TRUE.
Login to your guest account, there set chrome as your default browser (or even Firefox)
Then do not logout ! This would erase your session, so use the on the top right corner use the swich user drop down menue,
Now that you are back again in your Admin account,
Find the new temporary guest home folder, under
Select all the folders inside of it,
And copy them in :
English or whatever your languish is if you are using your computer in french this should be :
as you can see in this folder there was the defaults folders for the guest account
By modifying them you have just modified default appearance of the guest user account
You may want to save this folder in a safe place before, in case you want to undo your work.
For reference ,
You just have to follow the steps as shown in this video :
Or if you prefer text version :