css-layout-hack.js

// create a bookmark and use this code as the URL, you can now toggle the css on/off
// thanks+credit: https://dev.to/gajus/my-favorite-css-hack-32g3
javascript: (function() {
	var domStyle = document.createElement("style");
	domStyle.append(
			'* { color:#0f0!important;outline:solid #f00 1px!important; background-color: rgba(255,0,0,.2) !important; }\
            * * { background-color: rgba(0,255,0,.2) !important; }\
            * * * { background-color: rgba(0,0,255,.2) !important; }\
            * * * * { background-color: rgba(255,0,255,.2) !important; }\
            * * * * * { background-color: rgba(0,255,255,.2) !important; }\
            * * * * * * { background-color: rgba(255,255,0,.2) !important; }\
            * * * * * * * { background-color: rgba(255,0,0,.2) !important; }\
            * * * * * * * * { background-color: rgba(0,255,0,.2) !important; }\
            * * * * * * * * * { background-color: rgba(0,0,255,.2) !important; }'
	);
	document.body.appendChild(domStyle);
})();

I made a slight improvement. NOW you can trully turn it ON and OFF with the same bookmarklet on the fly (no need to reload)

javascript: (function() {
  let domStyle = document.getElementById('domStylee');
  if (domStyle) {
    document.body.removeChild(domStyle);
    return;
  }
  domStyle = document.createElement("style");
  domStyle.setAttribute('id', 'domStylee');
  domStyle.append(
    ['* { color:#0f0!important;outline:solid #f00 1px!important; background-color: rgba(255,0,0,.2) !important; }'],
    ['* * { background-color: rgba(0,255,0,.2) !important; }'],
    ['* * * { background-color: rgba(0,0,255,.2) !important; }'],
    ['* * * * { background-color: rgba(255,0,255,.2) !important; }'],
    ['* * * * * { background-color: rgba(0,255,255,.2) !important; }'],
    ['* * * * * * { background-color: rgba(255,255,0,.2) !important; }'],
    ['* * * * * * * { background-color: rgba(255,0,0,.2) !important; }'],
    ['* * * * * * * * { background-color: rgba(0,255,0,.2) !important; }'],
    ['* * * * * * * * * { background-color: rgba(0,0,255,.2) !important; }'].join()
  );
  document.body.appendChild(domStyle);
})();

I made a slight improvement. NOW you can trully turn it ON and OFF with the same bookmarklet on the fly (no need to reload)

javascript: (function() {
  let domStyle = document.getElementById('domStylee');
  if (domStyle) {
    document.body.removeChild(domStyle);
    return;
  }
  domStyle = document.createElement("style");
  domStyle.setAttribute('id', 'domStylee');
  domStyle.append(
    ['* { color:#0f0!important;outline:solid #f00 1px!important; background-color: rgba(255,0,0,.2) !important; }'],
    ['* * { background-color: rgba(0,255,0,.2) !important; }'],
    ['* * * { background-color: rgba(0,0,255,.2) !important; }'],
    ['* * * * { background-color: rgba(255,0,255,.2) !important; }'],
    ['* * * * * { background-color: rgba(0,255,255,.2) !important; }'],
    ['* * * * * * { background-color: rgba(255,255,0,.2) !important; }'],
    ['* * * * * * * { background-color: rgba(255,0,0,.2) !important; }'],
    ['* * * * * * * * { background-color: rgba(0,255,0,.2) !important; }'],
    ['* * * * * * * * * { background-color: rgba(0,0,255,.2) !important; }'].join()
  );
  document.body.appendChild(domStyle);
})();

this works perfectly in Chrome, but I couldn't get it to work in Firefox Dev Edition. Any ideas?