--- a/devtools/client/framework/toolbox.js
+++ b/devtools/client/framework/toolbox.js
@@ -7,16 +7,17 @@
 const SOURCE_MAP_WORKER = "resource://devtools/client/shared/source-map/worker.js";
 
 const MAX_ORDINAL = 99;
 const SPLITCONSOLE_ENABLED_PREF = "devtools.toolbox.splitconsoleEnabled";
 const SPLITCONSOLE_HEIGHT_PREF = "devtools.toolbox.splitconsoleHeight";
 const DISABLE_AUTOHIDE_PREF = "ui.popup.disable_autohide";
 const HOST_HISTOGRAM = "DEVTOOLS_TOOLBOX_HOST";
 const SCREENSIZE_HISTOGRAM = "DEVTOOLS_SCREEN_RESOLUTION_ENUMERATED_PER_USER";
+const CURRENT_THEME_SCALAR = "devtools.current_theme";
 const HTML_NS = "http://www.w3.org/1999/xhtml";
 
 var {Ci, Cu, Cc} = require("chrome");
 var promise = require("promise");
 var defer = require("devtools/shared/defer");
 var Services = require("Services");
 var {Task} = require("devtools/shared/task");
 var {gDevTools} = require("devtools/client/framework/devtools");
@@ -645,16 +646,21 @@ Toolbox.prototype = {
   },
 
   _pingTelemetry: function () {
     this._telemetry.toolOpened("toolbox");
 
     this._telemetry.logOncePerBrowserVersion(SCREENSIZE_HISTOGRAM,
                                              system.getScreenDimensions());
     this._telemetry.log(HOST_HISTOGRAM, this._getTelemetryHostId());
+
+    // Log current theme. The question we want to answer is:
+    // "What proportion of users use which themes?"
+    let currentTheme = Services.prefs.getCharPref("devtools.theme");
+    this._telemetry.logKeyedScalar(CURRENT_THEME_SCALAR, currentTheme, 1);
   },
 
   /**
    * Create a simple object to store the state of a toolbox button. The checked state of
    * a button can be updated arbitrarily outside of the scope of the toolbar and its
    * controllers. In order to simplify this interaction this object emits an
    * "updatechecked" event any time the isChecked value is updated, allowing any consuming
    * components to listen and respond to updates.

--- a/devtools/client/shared/telemetry.js
+++ b/devtools/client/shared/telemetry.js
@@ -12,16 +12,18 @@
 
 const TOOLS_OPENED_PREF = "devtools.telemetry.tools.opened.version";
 
 function Telemetry() {
   // Bind pretty much all functions so that callers do not need to.
   this.toolOpened = this.toolOpened.bind(this);
   this.toolClosed = this.toolClosed.bind(this);
   this.log = this.log.bind(this);
+  this.logScalar = this.logScalar.bind(this);
+  this.logKeyedScalar = this.logKeyedScalar.bind(this);
   this.logOncePerBrowserVersion = this.logOncePerBrowserVersion.bind(this);
   this.destroy = this.destroy.bind(this);
 
   this._timers = new Map();
 }
 
 module.exports = Telemetry;
 
@@ -305,16 +307,46 @@ Telemetry.prototype = {
       Services.telemetry.scalarSet(scalarId, value);
     } catch (e) {
       dump(`Warning: An attempt was made to write to the ${scalarId} ` +
            `scalar, which is not defined in Scalars.yaml\n`);
     }
   },
 
   /**
+   * Log a value to a keyed count scalar.
+   *
+   * @param  {String} scalarId
+   *         Scalar in which the data is to be stored.
+   * @param  {String} key
+   *         The key within the  scalar.
+   * @param  value
+   *         Value to store.
+   */
+  logKeyedScalar: function (scalarId, key, value) {
+    if (!scalarId) {
+      return;
+    }
+
+    try {
+      if (isNaN(value)) {
+        dump(`Warning: An attempt was made to write a non-numeric value ` +
+             `${value} to the ${scalarId} scalar. Only numeric values are ` +
+             `allowed.`);
+
+        return;
+      }
+      Services.telemetry.keyedScalarAdd(scalarId, key, value);
+    } catch (e) {
+      dump(`Warning: An attempt was made to write to the ${scalarId} ` +
+           `scalar, which is not defined in Scalars.yaml\n`);
+    }
+  },
+
+  /**
    * Log a value to a keyed histogram.
    *
    * @param  {String} histogramId
    *         Histogram in which the data is to be stored.
    * @param  {String} key
    *         The key within the single histogram.
    * @param  [value]
    *         Optional value to store.

--- a/toolkit/components/telemetry/Scalars.yaml
+++ b/toolkit/components/telemetry/Scalars.yaml
@@ -770,16 +770,31 @@ devtools.grid.showInfiniteLines:
     expires: never
     kind: uint
     notification_emails:
       - dev-developer-tools@lists.mozilla.org
     release_channel_collection: opt-out
     record_in_processes:
       - 'main'
 
+devtools:
+  current_theme:
+    bug_numbers:
+      - 1396811
+    description: >
+      Number of times DevTools was opened, keyed by theme.
+    expires: never
+    kind: uint
+    keyed: true
+    notification_emails:
+      - dev-developer-tools@lists.mozilla.org
+    release_channel_collection: opt-out
+    record_in_processes:
+      - 'main'
+
 navigator.storage:
   estimate_count:
     bug_numbers:
       - 1359708
     description: >
       Number of times navigator.storage.estimate has been used.
     expires: "60"
     kind: uint