Style Sheets

Since Gaphor 2.0, diagrams can have a different look by means of style sheets. Style sheets use the Cascading Style Sheets (CSS) syntax. CSS is used to describe the presentation of a document written in a markup language, and is most commonly used with HTML for web pages.

On the W3C CSS home page, CSS is described as:

Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g., fonts, colors, spacing) to Web documents.

Its application goes well beyond web documents, though. Gaphor uses CSS to provide style elements to items in diagrams. CSS allows us, users of Gaphor, to change the visual appearance of our diagrams. Color and line styles can be changed to make it easier to read the diagrams.

Since we’re dealing with a diagram, and not a HTML document, some CSS features have been left out.

The style is part of the model, so everyone working on a model will have the same style.

Here is a simple example of how to change the background color of a class:

class {
  background-color: beige;

background beige

Or change the color of a component, only when it’s nested in a node:

node component {
  background-color: skyblue;

nested component

The diagram itself is also expressed as a CSS node. It’s pretty easy to define a “dark” style:

diagram {
  background-color: black;

* {
  color: white;
  text-color: white;

dark style

Here you already see the first custom attribute: text-color. This property allows you to control the color of the text drawn in an item. color is used for the lines (strokes) that make the layout of a diagram item.

Supported selectors

Since we are dealing with diagrams and models, we do not need all the features of CSS. Below you’ll find a summary of all CSS features supported by Gaphor.


All items on the diagram, including the diagram itself.

node component

Any component item which is a descendant of a node.

node > component

A component item which is a child of a node.


A generalization item with a subject present.


A class with name “Foo”.


A diagram with a name starting with “draft”.


A diagram with a name ends with “draft”.


A diagram with a name containing the text “draft”.

diagram[name~=draft item]

A diagram with a name of “draft” or “item”.


A diagram with a name is “draft” or starts with “draft-“.


The focused item. Other pseudo classes are:

  • :active selected items

  • :hover for the item under the mouse

  • :drop if an item is dragged and can be dropped on this item


A node containing no child nodes in the diagram.


An item is at the top level of the diagram.

This is only applicable for the diagram


The item contains any of the provided selectors.

E.g. node:has(component): a node containing a component item.


Match any of the provided selectors.

E.g. :is(node, subsystem) > component: a node or subsystem.


Negate the selector.

E.g. :not([subject]): Any item that has no “subject”.

  • The official specification of CSS3 attribute selectors.

  • Gaphor provides the |= attribute selector for the sake of completeness. It’s probably not very useful in this context, though.

  • Please note that Gaphor CSS does not support IDs for diagram items, so the CSS syntax for IDs (#some-id) is not used. Also, class syntax (.some-class) is not supported currently.

Style properties

Gaphor supports a subset of CSS properties and some Gaphor specific properties. The style sheet interpreter is relatively straight forward. All widths, heights, and sizes are measured in pixels. You can’t use complex style declarations, like the font property in HTML/CSS which can contain font family, size, weight.




background-color: azure;

background-color: rgb(255, 255, 255);

background-color: hsl(130, 95%, 10%);


Color used for lines


Color for text


Color opacity factor (0.0 - 1.0), applied to all colors

  • A color can be any CSS3 color code, as described in the CSS documentation. Gaphor supports all color notations: rgb(), rgba(), hsl(), hsla(), Hex code (#ffffff) and color names.

Text and fonts


A single font name (e.g. sans, serif, courier)


An absolute size (e.g. 14) or a size value (e.g. small)


Either normal or italic


Either normal or bold


Either left, center, right


Either none or underline


Vertical alignment for text

Either top, middle or bottom


Set vertical spacing for icon-like items (actors, start state)

Example: vertical-spacing: 4

  • font-family can be only one font name, not a list of (fallback) names, as is used for HTML.

  • font-size can be a number or CSS absolute-size values. Only the values x-small, small, medium, large and x-large are supported.

Drawing and spacing


Radius for rectangles: border-radius: 4


Style for dashed lines: dash-style: 7 5


Either normal or sloppy [factor]


Set the width for lines: line-width: 2


Set minimal height for an item: min-height: 50


Set minimal width for an item: min-width: 100


CSS style padding (top, right, bottom, left)

Example: padding: 3 4

  • padding is defined by integers in the range of 1 to 4. No unit (px, pt, em) needs to be used. All values are in pixel distance.

  • dash-style is a list of numbers (line, gap, line, gap, …)

  • line-style only has an effect when defined on a diagram. A sloppiness factor can be provided in the range of -2 to 2.

Diagram styles

Only a few properties can be defined on a diagram, namely background-color and line-style. You define the diagram style separately from the diagram item styles. That way it’s possible to set the background color for diagrams specifically. The line style can be the normal straight lines, or a more playful “sloppy” style. For the sloppy style an optional wobliness factor can be provided to set the level of line wobbliness. 0.5 is default, 0.0 is a straight line. The value should be between -2.0 and 2.0. Values between 0.0 and 0.5 make for a subtle effect.


Here are some ideas that go just beyond changing a color or a font. With the following examples we dig in to Gaphor’s model structure to reveal more information to the users.

To create your own expression you may want to use the Console (Tools -> Console, in the Hamburger menu). Drop us a line on Gitter and we would be happy to help you.

The drafts package

All diagrams in the package “Drafts” should be drawn using sloppy lines:

diagram[] {
  line-style: sloppy 0.3;

diagram[name=draft] * {
  font-family: Purisa; /* Or use some other font that's installed on your system */

draft style

Unconnected relationships

All items on a diagram that are not backed by a model element, should be drawn in a dark red color. This can be used to spot not-so-well connected relationships, such as Generalization, Implementation, and Dependency. These items will only be backed by a model element once you connect both line ends. This rule will exclude simple elements, like lines and boxes, which will never have a backing model element.

:not([subject], :is(line, box, ellipse, commentline)) {
  color: firebrick;

unconnected relationship