diff --git a/core/block_svg.js b/core/block_svg.js
index e5c55a457..1d1d57835 100644
--- a/core/block_svg.js
+++ b/core/block_svg.js
@@ -156,16 +156,6 @@ Blockly.BlockSvg.INLINE = -1;
  */
 Blockly.BlockSvg.COLLAPSED_WARNING_ID = 'TEMP_COLLAPSED_WARNING_';
 
-// Leftover UI constants from block_render_svg.js.
-
-/**
- * Do blocks with no previous or output connections have a 'hat' on top?
- * @const
- * @package
- */
-// TODO (#3142): Remove.
-Blockly.BlockSvg.START_HAT = false;
-
 /**
  * An optional method called when a mutator dialog is first opened.
  * This function must create and initialize a top-level block for the mutator
diff --git a/core/renderers/common/constants.js b/core/renderers/common/constants.js
index 4956188ea..35c3f3d7f 100644
--- a/core/renderers/common/constants.js
+++ b/core/renderers/common/constants.js
@@ -192,6 +192,13 @@ Blockly.blockRendering.ConstantProvider = function() {
    */
   this.MAX_BOTTOM_WIDTH = 66.5;
 
+  /**
+   * Whether to add a 'hat' on top of all blocks with no previous or output
+   * connections. Can be overridden by 'hat' property on Theme.BlockStyle.
+   * @type {boolean}
+   */
+  this.ADD_START_HATS = false;
+
   /**
    * Height of the top hat.
    * @private
diff --git a/core/renderers/common/info.js b/core/renderers/common/info.js
index 460cee391..94ac6b8ad 100644
--- a/core/renderers/common/info.js
+++ b/core/renderers/common/info.js
@@ -262,8 +262,8 @@ Blockly.blockRendering.RenderInfo.prototype.createRows_ = function() {
  */
 Blockly.blockRendering.RenderInfo.prototype.populateTopRow_ = function() {
   var hasPrevious = !!this.block_.previousConnection;
-  var hasHat = (this.block_.hat ?
-    this.block_.hat === 'cap' : Blockly.BlockSvg.START_HAT) &&
+  var hasHat = (typeof this.block_.hat !== 'undefined' ?
+    this.block_.hat === 'cap' : this.constants_.ADD_START_HATS) &&
     !this.outputConnection && !hasPrevious;
   var leftSquareCorner = this.topRow.hasLeftSquareCorner(this.block_);
 
diff --git a/core/renderers/measurables/rows.js b/core/renderers/measurables/rows.js
index 1cf09ddcf..6a86f3fa4 100644
--- a/core/renderers/measurables/rows.js
+++ b/core/renderers/measurables/rows.js
@@ -290,8 +290,9 @@ Blockly.utils.object.inherits(Blockly.blockRendering.TopRow,
  * @return {boolean} Whether or not the top row has a left square corner.
  */
 Blockly.blockRendering.TopRow.prototype.hasLeftSquareCorner = function(block) {
-  var hasHat = (block.hat ? block.hat === 'cap' : Blockly.BlockSvg.START_HAT) &&
-    !block.outputConnection && !block.previousConnection;
+  var hasHat = (typeof block.hat !== 'undefined' ?
+      block.hat === 'cap' : this.constants_.ADD_START_HATS) &&
+      !block.outputConnection && !block.previousConnection;
   var prevBlock = block.getPreviousBlock();
 
   return !!block.outputConnection ||