Monokai. It's the same theme I use in every editor. It's my preferred scheme.

I use atom-material The link is to the github repo that contains syntax and design.

I'm curious if it has gotten PHP IDE functionality which PHPStorm is almost exclusive to have?

I tweaked my UI quite a bit. The syntax theme I use is called Resonance apm i resonance-syntax. For the editor theme, I use Atom Material apm i atom-material-ui

On macOS/Linux I use custom title bar (on Windows it's not supported by Electron as of yet). Atom has a great feature to color the tab bar and together with custom title bar I now get an eye appealing editor. Easy to spot on my 34" display but not eye hurting - at least to me.

Here is an excerpt of my config.

"*"
  core: 
    useCustomTitleBar: true
  editor:
    fontFamily: "\"OperatorMono-XLight\", \"Fantasque Sans Mono\""
    fontSize: 18
    showInvisibles: true
    zoomFontWhenCtrlScrolling: true
  atom-material-ui:
    colors:
      abaseColor: "#ffd479"
      accentColor: "#92b5ff"
      genAccent: true
    tabs:
      compactTabs: true
      tintedTabBar: true
    treeView:
      blendTabs: true
    ui:
      panelShadows: false

Oceanic Next Italic, It just doesn't get any better than that for me. Perfect balance between contrast, readability and looks.