I am not a fan of tweaking an operating system.
This can cause you no end of problems if you are not extremely sure of what you are doing.
And even if you are sure, the next OS update can be a problem, or cause a problem.
Having said that,
Tinker Tool will allow you to make many customizations in a relatively safe manner.
One "must have" tweak which you can make with TinkerTool is on the Safari tab.
Check the box for "Redirect all links opening new windows into foreground tabs".
This fixes one of the most irritating flaws in Safari.
I did make one manual customization to my Mac.
I gave it a custom startup screen.
If you wish to do this, you replace the jpeg file at this location:
/System/Library/CoreServices/DefaultDesktop.jpg