Forum Discussion
Removing the Quick Launch & Keeping the Global Navigation Bar in SP Online to max Screen Real estate
#sideNavBox { display:none !important }
#contentBox { margin-left:20px !important; }
Those two lines are all you need, you just need a way to get them injected into the entire site.
#sideNavBox { display:none !important }
#contentBox { margin-left:20px !important; }
It is common for non Front-enders to whack in ``!important`` because they are fighting CSS load order.
But you have now hijacked the DOM elements and no other App or CSS style can easily restore them.
Just increase Specificity so it doesn't matter in which order your CSS is applied
#sideNavBox#sideNavBox { display:none}
#contentBox#contentBox { margin-left:20px}
By adding even more Specificity any other code can still override you changes
After Selectors:
https://code.tutsplus.com/tutorials/the-30-css-selectors-you-must-memorize--net-16048
Specificity is the most powerfull CSS concept:
https://www.smashingmagazine.com/2007/07/css-specificity-things-you-should-know/