Raw Source
giocondo.sticca / GMail New UI improved 

// ==UserScript==
// @name          	GMail New UI improved
// @description     Improvements to the new GMail UI
// @author			Giocondo Sticca <giocondo.sticca@gmail.com>
// @namespace       https://github.com/giocondo
// @match			https://mail.google.com/mail/*
// @copyright       2018, Giocondo Sticca
// @license         MIT
// @version         1.0
// @run-at			document-start
// ==/UserScript==

var hnd=setInterval(function(){
  
  var s=document.createElement('style');
  var t=document.createTextNode('* { font-family: Arial !important; }'+
                                '.z0>.L3 { border-radius: 5px !important; font-weight: bold !important; color: white; margin-left: 20px; background-color: #D93025; padding: 0px 30px !important; height: 38px; text-transform: uppercase !important; box-shadow: unset; }'+
                                '.z0>.L3::before { display: none; }'+
                                '.z0>.L3:hover,.z0>.L3:active,.z0>.L3:focus { background-color: #c44332 !important; transition: unset !important; box-shadow: unset !important; }'+
                                '.yO { background: #f4f4f4; }'+
                                '.Am, .LW-avf.Am { font-size: 16px !important;}'+
                                '.TK .TO:active, .n6 .ah9.aiu:active, .CL:active, .TO.nZ, .TO.ol { background-color: white; }'+
                                '.wT>.n3 .byl:first-child .aim:first-child .nZ { background-color: unset; }' +
                                'header.gb_Ta { background-color: #F1F1F1 !important; border-bottom: 1px solid #e5e5e5; }'+
                                '.x7 { background: #fdfaef !important; }'+
                                '.x7:hover { background: #f5f2e7 !important; }'+
                                '.a3s, .g6 { font: 14px/1.5 Arial,Helvetica,sans-serif !important; }'+
                                '.zE { background: #f4f4f4; }'+                               
                                '#aso_search_form_anchor { background-color: white; border: 1px solid #aaa; }'+
                                'header { background-color: #F1F1F1; }'+
                                '.TC { font-size: 85%; }'+
                                '.SK { border: 1px solid #AAA; }'+
                                '.aqN { display: none; }'+
                                '.zA { padding-top: 9px !important; padding-bottom: 9px !important; }'+
                                '.zA:hover { background-color: #eee; border-bottom-color: #E2E5E7; box-shadow: inset 0 -1px 0 0 rgba(100,121,143,0.122); }'+
                                '.gb_Se .gb_qf { border: 1px solid #aaa; background-color: white; }'+
                                
                                '.bhZ:not(.bym) .z0>.L3 { background-color: transparent !important; color: transparent !important; }'+
                                
                                '.bhZ:not(.bym) .z0>.L3::after { background-color: #D93025; width: 30px; height: 30px; content: "sf";'+
                                ' position: absolute; top: 9px; left: -6px; border-radius: 5px; content: "+"; color: white;'+
                                'font-size: 30px; line-height: 32px; text-align: center;}'+
                                
                                '.bhZ:not(.bym) .z0>.L3:hover::after { background-color: #c44332;}'+
                                
                                '.amH>.Dj, .h0>.adl { font-size: 1rem !important; }'); 
  s.appendChild(t);
  try { document.getElementsByTagName('body')[0].appendChild(s); } catch(e){ return; }

  clearInterval(hnd);

},10);