Atom Editor One Dark theme modifications for OS X.

styles.less

// Hide icons in tabs
.tab-bar li.tab .title:before {
  display: none;
}

// Left align close button and modified indicator in tabs
.tab-bar .tab.active .title,
.tab-bar .tab .title {
  padding-left: 28px;
}
.tab-bar .tab .close-icon {
  right: initial;
  margin-right: auto;
  margin-left: 10px;
}
.tab-bar .tab.modified:not(:hover) .close-icon {
  right: initial;
}

// Size tabs to fit available space
.tab-bar .tab.active[data-type="TextEditor"],
.tab-bar .tab {
  max-width: none;
  min-width: initial;
  width: auto;
  padding: 0;
}
.tab-bar .tab.active {
  flex: 1;
}

// Don't show blue indicator in an empty tab-bar
.tab-bar:empty:before {
  background: transparent;
}

Now an Atom Editor package: https://atom.io/packages/close-on-left.