Themes are a way to customize the look and feel of Blockly. You can customize block colours, category colours and certain components through the Themes class. If you want to customize additional components, we provide the theme name as a class on the injection div. This allows users to change the look of any unsupported components through CSS. Our main goal in creating themes is to empower developers to create Blockly experiences that are more accessible.

Theme Properties

A theme is an object with several properties: the base theme to extend from, a style object for blocks, categories, components, and fonts, and a whether or not to use hats for start blocks.

Block Style

A block style is made up of four fields:

  • colourPrimary (required) - The background colour of the block.
  • colourSecondary (optional) - The background colour if the block is a shadow block.
  • colourTertiary (optional) - The border or highlight colour of the block.
  • hat (optional) - Adds a hat to a block, if the value is set to cap. Learn more about hats.

Block with arrows pointing to primary, secondary and tertiary colour

const listBlocks = {
   'colourPrimary': '#4a148c',
   'hat': 'cap'

A theme will include a mapping of block style name to block style object:

const blockStyles = {
   'list_blocks': {
      'colourPrimary': '#4a148c',
   'logic_blocks': {
      'colourPrimary': '#01579b',

Category Style

A category style only holds a colour property.

  • Colour (required) - The colour of the category in the toolbox. Usually these colours should be the same as the colourPrimary on the majority of blocks in the category. This makes it easy for users to tell which blocks belong in a given category.

Screenshot of toolbox with different category colours

const mathCategory = {

A theme will include a mapping of category name to category style object:

const categoryStyles = {
   'list_category': {
      'colour': '#4a148c'
   'logic_category': {
      'colour': '#01579b',

Component Styles

A theme can set the colour or value of the below components:

  • workspaceBackgroundColour: The workspace background colour
  • toolboxBackgroundColour: Toolbox background colour
  • toolboxForegroundColour: Toolbox category text colour
  • flyoutBackgroundColour: Flyout background colour
  • flyoutForegroundColour: Flyout label text colour
  • flyoutOpacity: Flyout opacity
  • scrollbarColour: Scrollbar colour
  • scrollbarOpacity: Scrollbar opacity
  • insertionMarkerColour: The insertion marker colour (Does not accept colour names)
  • insertionMarkerOpacity: The insertion marker opacity
  • markerColour: The colour of the marker displayed in keyboard navigation mode
  • cursorColour: The colour of the cursor displayed in keyboard navigation mode

Most other components can be changed by using the theme name in your CSS. However, if there is a component you would like to change that is not already a part of this list and cannot be changed using CSS, please file an issue with more information.

const componentStyle = {
   'workspaceBackgroundColour': '#1e1e1e',
   'toolboxBackgroundColour': '#333'

Font Styles

A font style is an object that holds the family, weight and size of a font.

const fontStyle = {
   'family': 'Georgia, serif',
   'weight': 'bold',
   'size': 12

Start Hats

If you set startHats: true directly in a theme object, a hat will be added to all blocks with no previous or output connections. If you want more control over exactly which blocks get a hat, you can instead use the block style property.

Custom Themes

In order to add a theme to your Blockly application, you will need to do the following:

  1. Create a theme
  2. Add style names
  3. Set your theme on a workspace

Create a Theme

A theme can be created using the constructor or by using defineTheme. Using defineTheme makes it easy to extend a pre existing theme and set all values with a single object. A theme has a name, and is an object with the block styles, category styles, and other properties as discussed above.

A theme can also have a base theme, which will provide default values for any values not specified in the custom theme.

const theme = Blockly.Theme.defineTheme('themeName', {
   'base': Blockly.Themes.Classic,
   'blockStyles': {
      'logic_blocks': {
         'colourPrimary': '#4a148c'
      'math_blocks': {...}
   'categoryStyles': {...},
   'componentStyles': {...},
   'fontStyle': {...},
   'startHats': true

Here is an example of using defineTheme.

Add Style Names

Now that we have created a theme we need to add the name of the styles to the block and category definitions.


How you define your toolbox determines how you need to add the style name. See the toolbox page for more information.


How you define your block determines how you need to add the style name. You can find more on block definitions on our custom blocks page. You will assign one of the block styles that you defined in the theme's blockStyles property to your block.





Set your Theme

You also need to tell Blockly which theme to use. By defining several themes that use the same block style and category names, you can allow the user to choose the theme that works best for them, and change it dynamically without having to change the block definitions themselves.

Initial theme

The best way to set an initial theme is by including options.theme in your inject call. You can supply the theme in JSON or JavaScript.


   theme: {
      "blockStyles" : {
         "list_blocks": {
            "colourPrimary": "#4a148c",
      "categoryStyles" : {
         "list_category": {
            "colour": "#4a148c"
      "componentStyles" : {
         "workspaceBackgroundColour": "#1e1e1e"


   theme: Blockly.Theme.defineTheme('themeName', {
      'blockStyles': blockStyles,
      'categoryStyles': categoryStyles,
      'componentStyles': componentStyles

More information on options can be found on our configuration page. If no theme is provided then it will default to the Classic Theme.

Dynamic themes

If you want to dynamically change your theme (for instance in the case of allowing users to choose a theme from a dropdown menu) then you can call yourWorkspace.setTheme(theme).

Create Block Styles Script

Blockly provides a script that will take in a map of hues or hex values and will calculate the secondary and tertiary colours for them. The script can be found in the scripts/themes folder.

Built-in themes

Blockly provides a number of themes for accessibility, specifically certain types of colour vision deficiency:

We also have: