CLI elements can be styled via a CSS-like language of selectors and properties. Only a small subset of CSS3 is supported, and a lot visual properties cannot be implemented on a terminal, so these will be ignored as well.

The style of an element is calculated from themes from four sources. These form a stack, and the themes on the top of the stack take precedence, over themes in the bottom.

1. The cli package has a built-in theme. This is always active. See builtin_theme().

2. When an app object is created via start_app(), the caller can specify a theme, that is added to theme stack. If no theme is specified for start_app(), the content of the cli.theme option is used. Removed when the corresponding app stops.

3. The user may specify a theme in the cli.user_theme option. This is added to the stack after the app's theme (step 2.), so it can override its settings. Removed when the app that added it stops.

4. Themes specified explicitly in cli_div() elements. These are removed from the theme stack, when the corresponding cli_div() elements are closed.

## Writing themes

A theme is a named list of lists. The name of each entry is a CSS selector. Only a subset of CSS is supported:

• Type selectors, e.g. input selects all <input> elements.

• Class selectors, e.g. .index selects any element that has a class of "index".

• ID selector. #toc will match the element that has the ID "toc".

• The descendant combinator, i.e. the space, that selects nodes that are descendants of the first element. E.g. div span will match all <span> elements that are inside a <div> element.

The content of a theme list entry is another named list, where the names are CSS properties, e.g. color, or font-weight or margin-left, and the list entries themselves define the values of the properties. See builtin_theme() and simple_theme() for examples.

## Formatter callbacks

For flexibility, themes may also define formatter functions, with property name fmt. These will be called once the other styles are applied to an element. They are only called on elements that produce output, i.e. not on container elements.

## Supported properties

Right now only a limited set of properties are supported. These include left, right, top and bottom margins, background and foreground colors, bold and italic fonts, underlined text. The before and after properties are supported to insert text before and after the content of the element.

The current list of properties:

• after: A string literal to insert after the element. It can also be a function that returns a string literal. Supported by all inline elements, list items, alerts and rules.

• background-color: An R color name, or HTML hexadecimal color. It can be applied to most elements (inline elements, rules, text, etc.), but the background of containers is not colored properly currently.

• before: A string literal to insert before the element. It can also be a function that returns a string literal. Supported by all inline elements, list items, alerts and rules.

• class-map: Its value can be a named list, and it specifies how R (S3) class names are mapped to cli class names. E.g. list(fs_path = "file") specifies that fs_path objects (from the fs package) should always print as .file objects in cli.

• color: Text color, an R color name or a HTML hexadecimal color. It can be applied to most elements that are printed.

• collapse: Specifies how to collapse a vector, before applying styling. If a character string, then that is used as the separator. If a function, then it is called, with the vector as the only argument.

• digits: Number of digits after the decimal point for numeric inline element of class .val.

• fmt: Generic formatter function that takes an input text and returns formatted text. Can be applied to most elements. If colors are in use, the input text provided to fmt already includes ANSI sequences.

• font-style: If "italic" then the text is printed as cursive.

• font-weight: If "bold", then the text is printed in boldface.

• line-type: Line type for cli_rule().

• list-style-type: String literal or functions that returns a string literal, to be used as a list item marker in un-ordered lists.

• margin-bottom, margin-left, margin-right, margin-top: Margins.

• padding-left, padding-right: This is currently used the same way as the margins, but this might change later.

• start: Integer number, the first element in an ordered list.

• string_quote: Quoting character for inline elements of class .val.

• text-decoration: If "underline", then underlined text is created.

• text-exdent: Amount of indentation from the second line of wrapped text.

• transform: A function to call on glue substitutions, before collapsing them. Note that transform is applied prior to implementing color via ANSI sequences.

• vec_last: The last separator when collapsing vectors.

• vec_sep: The separator to use when collapsing vectors.

• vec_trunc: Vectors longer than this will be truncated. Defaults to 100.

More properties might be added later. If you think that a property is not applied properly to an element, please open an issue about it in the cli issue tracker.

## Examples

Color of headings, that are only active in paragraphs with an 'output' class:

Create a custom alert type: