CSS Patterns

This document describes the patterns we are using to organize and write CSS for JupyterLab.

CSS checklist

CSS classnames are defined as all caps file-level consts and follow the CSS class naming convention described below.
CSS files for plugins are located within the plugin and imported into the plugin's index.css.
The JupyterLab default CSS variables in src/default-theme/variables.css are used to style plugins where ever possible.
Additional public/private CSS variables are defined by plugins sparingly and in accordance with the conventions described below.

CSS variables

We are using native CSS variables in JupyterLab. This is to enable dynamic theming of built-in and third party plugins. As of Septmeber 2016, CSS variables are supported in the latest stable versions of all popular browsers, except for IE/Edge. If a JupyterLab deployment needs to support these browsers, a server side CSS preprocessor such as Myth or cssnext may be used.

Naming of CSS variables

We use the following convention for naming CSS variables:

Start all CSS variables with --jp-
Words in the variable name should be lowercase and separated with -
The final segment will typically be related to a CSS properties, such as color, font-size or background.
Intermediate segments should have refer to the component and subcomponent, such as --jp-notebook-cell-.

Public/private

Some CSS variables in JupyterLab are considered part of our public API. Others are considered private and should not be used by third party plugins or themes. The difference between public and private variables is simple:

All private variables begin with --jp-private-
All variables without the private- prefix are public.
Public variables should be defined under the :root pseudo-selector. This ensures that public CSS variables can be inspected under the top-level <html> tag in the browser's dev tools.
Where possible, private variables should be defined and scoped under an appropriate selector.

CSS variable usage

JupyterLab includes a default set of CSS variables in the file:

src/default-theme/variables.css

To ensure consistent design in JupyterLab, all built-in and third party extensions should use these variables in their styles if at all possible. Documentation about those variables can be found in the variables.css file itself.

Plugins are free to define additional public and private CSS variables in their own index.css file, but should do so sparingly.

File organization

We are organizing our CSS files in the following manner:

Each plugin/subdirectory in the top-level src directory should contain any CSS files needed to style itself.
Multiple CSS files may be used and organized as needed, but they should be imported into a single index.css at the top-level of the plugin.
The index.css of each plugin should be imported inside src/default-theme/index.css.

CSS class names

We have a fairly formal method for naming our CSS classes.

First, CSS class names are associated with TypeScript classes that extend phosphor.Widget:

The .node of each such widget should have a CSS class that matches the name of the TypeScript class, and those classnames should be defined as a file-level const:

const MYWIDGET_CLASS = 'jp-MyWidget';

class MyWidget extends Widget {

  constructor() {
    super();
    this.addClass(MYWIDGET_CLASS);
  }

}

Second, subclasses should have a CSS class for both the parent and child:

const MYWIDGET_SUBCLASS_CLASS = 'jp-MyWidgetSubclass';

class MyWidgetSubclass extends MyWidget {

  constructor() {
    super(); // Adds `jp-MyWidget`
    this.addClass(MYWIDGET_SUBCLASS_CLASS);
  }

}

Third, children nodes of a Widget should have a third segment in the CSS class name that gives a semantic naming of the component, such as:

jp-MyWidget-toolbar
jp-MyWidget-button
jp-MyWidget-contentButton

Thus, the general naming of CSS classes is of the form jp-WidgetName-semanticChild.

Fourth, some CSS classes are used to modify the state of a widget:

jp-mod-active: applied to elements in the active state
jp-mod-hover: applied to elements in the hover state
jp-mod-selected: applied to elements while selected

Fifth, some CSS classes are used to distinguish different types of a widget:

--jp-type-separator: applied to menu items that are separators
--jp-type-directory: applied to elements in the file browser that are directories

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