For use with the jenkins `simple theme` plugin, this js modifies the left sidebar. It hides left nav links and moves some sections to the bottom.

jenkins-custom-theme.js

function customJenkins() {
  var url = window.location.href.replace(window.location.protocol, ''), 
    urls = ['//127.0.0.1:8080/','//jenkins:8080/','//jenkins:8080/view/all/builds','//127.0.0.1:8080/view/all/builds'];
  if (urls.includes(url)) { 
    // hide most of the left nav links on the pages in the urls array
    var dontHide = ['Manage Jenkins','Build History','New Item'];
    try {
      var tasks = document.getElementsByClassName('task');
      for (var i = 0; i < tasks.length; i++) {
        if (!dontHide.includes(tasks[i].innerText)) {
          tasks[i].style.display = 'none';
        }
      }
      // Expand 'next executions' and 'executor status'
      var expandThese = ['next-exec','executors'];
      for (var j = 0; j < expandThese.length; j++) {
        var expandThis = document.getElementById(expandThese[j]);
        if(typeof(expandThis) !== 'undefined' && expandThis !== null) {
          var el = expandThis.children[0].children[0].children[1];
          if (el.getAttribute('title') === 'expand') {  
            el.click();
          } else {
            console.log(expandThese[j] + ' is not expandable');
          }
        } else {
          console.log('jenkins-custom-theme.js no element found with id ' + expandThese[j]);
        }
      }
    } catch (e) {
      console.error({'jenkins-custom-theme.js error:' : e});
    }
  } else {
    console.warn('jenkins-custom-theme.js is not configured for this url.');
  }
  // Move Build Queue to the bottom of the side-panel
  var sidePanel = document.getElementById('side-panel'), buildQueue = document.getElementById('buildQueue');
  if (typeof(sidePanel) !== 'undefined' && sidePanel !== null && typeof(buildQueue) !== 'undefined' && buildQueue !== null ) {
    buildQueue.remove();
    buildQueue = '<div id="buildQueue" class="container-fluid pane-frame track-mouse collapsed">' + buildQueue.innerHTML + '</div>';
    sidePanel.insertAdjacentHTML('beforeEnd', buildQueue);
  }
}

if (window.addEventListener) {
  window.addEventListener('load', customJenkins, false);
  console.info('addEventListener customJenkins()');
} else if (window.attachEvent) {
  window.attachEvent('onload', customJenkins );
  console.info  ('attachEvent customJenkins()');
}