In situ Syntax Highlighting in macOS Applications Like Keynote

Earlier this month, I returned to CSS Day for the first time since 2018 to deliver my first in-person talk since 2022.  “Forging Our Own Paths” should be available at some point; in the meantime, for the six or seven people in my audience who might need to do something similar, I’d like to share a small macOS workflow I developed to make syntax-highlighting code blocks in situ in Keynote a lot simpler.  The end result is to have an entry (or entries) in the Services submenu of the contextual (right-click) menu for highlighted text.  All this is adapted from an old blog post I found copied in a few places, and which needed some updates to make things work in 2026.

First, install highlight.  I used brew install highlight, and the rest of this piece assumes you’ve done it that way.  If you install it another way, such that it ends up in a different location than Homebrew would give it, you’ll need to modify a variable value later on, which I’ll point out when we get there.

Next, you need to install the following (also available as a gist) as a shell script called keynote-highlight:

#!  /bin/bash
set -e
while getopts 'h:o:i:s:t:' OPTION; do
	case "$OPTION" in
		h)
			highlighthome="$OPTARG"
			;;
		o)
			outputrtf="$OPTARG"
			;;
		i)
			inputrtf="$OPTARG"
			;;
		s)
			syntax="$OPTARG"
			;;
		t)
			theme="$OPTARG"
			;;
		?)
			echo "script usage incorrect?" >&2
			exit 1
			;;
	esac
done
shift "$(($OPTIND -1))"

#=============================

inputrtf="$(pbpaste -pboard -prefer public.rtf)"

regex="fcharset0 ([a-zA-Z0-9 ]+);"
if [[ "$inputrtf" =~ $regex ]]
then
	fontface=${BASH_REMATCH[1]}
else
	fontface="Courier"
fi

regex="fs([0-9]{1,5})"
if [[ "$inputrtf" =~ $regex ]]
then
	fontsize=${BASH_REMATCH[1]}
	fontsize2=$((fontsize/2))
else
	fontsize2="12"
fi

if [ -z "$theme" ]; then
	theme="candy"
fi

if [ -z "$highlighthome" ]; then
	highlighthome="/opt/homebrew/bin/highlight"
fi

highlighted=$("$highlighthome" --out-format="rtf" --syntax="$syntax" --style="$theme" --font="$fontface" --font-size="$fontsize2" --no-trailing-nl --stdout)
echo "$highlighted"

Put the script wherever you store your shell scripts, and make sure it’s both executable and can be invoked from the command line.  I believe, without any real basis for doing so, that if you already have syntax-highlight installed, which is (among other things) a wrapper around highlight, you could use it by modifying the highlighthome variable assignment to point to it rather than highlight, as well as modifying a variable in an upcoming bit of code.  But, as I say, I’m just guessing about that.

Once the shell script is installed and ready to execute, launch Automator and create a new Quick Action.  Call it “Syntax Highlight CSS” or something similar.  If you want to set up highlighting for other kinds of code, like HTML or any of the nearly 250 languages (!!!) highlight supports, each language has to be given its own Quick Action.  Thus, if you want them all next to each other in the Services menu, pick an appropriate naming scheme.  For this one, we’re doing CSS, but later you’ll see how you can quickly set up this same thing for other formats.

At the top of the right-hand panel in the new Quick Action workflow, check the “Workflow receives current” dropdown to make sure it’s set to either “Automatic (rich text)” or “rich text”, the latter if you plan to never, ever use this in any non-RTF setting.  I go with the Automatic option.  If you want to restrict the action to a particular application, like Keynote, change the dropdown that says “any application” to pick a specific application.  I leave mine to be available in any application, just in case I’m ever syntax highlighting code in TextEdit or something.  I also set the color to “Red”, because clearly that makes it go faster.

With all those things set, the first thing to add to the workflow is a “Copy to Clipboard” action.  That’s it for this step, just add that and leave it alone.

Now, add a “Run AppleScript” action.  Paste the following (also available as a gist) into the textbox that contains the boilerplate skeleton (replace the skeleton):

on run {input, parameters}
	set highlightHome to "/opt/homebrew/bin/highlight"
	set syntaxType to "css"
	set themeName to "navy"
	set command to "PATH_TO_SCRIPT/keynote-highlight -h " & highlightHome & " -s " & syntaxType & " -t " & themeName
	do shell script "/bin/bash -c 'pbpaste | " & command & " | pbcopy'"
	delay 0.1
	tell application "System Events" to keystroke "v" using command down
end run

Change the PATH_TO_SCRIPT in there to wherever you put the shell script, save the workflow, and it should be ready to go!

…unless your copy of highlight lives somewhere else or you’re trying out using syntax-highlight in its place, or you have a different theme you’d like to use.  In either case, change the value of the corresponding variable in the AppleScript.  As for the syntaxType variable, that’s what you change if you want to highlight HTML or Pascal or BASIC or whatever else, but since we’re doing CSS, leave it as is.

At this point, everything should be ready to go.  In your Keynote slides, wherever you want to syntax-highlight some CSS, drag-select (or select-all) the CSS text in question.  Just be sure you have the text actually highlighted; just selecting the outer text box that holds the text isn’t sufficient.  Right-click on the selected text to bring up the Context menu, and in there open the “Services” submenu.  “Syntax Highlight CSS” (or whatever you called yours) should be in that submenu.  Select it, and after a second or two, the un-highlighted CSS should be replaced with the same thing, except syntax-highlighted.

Well, “the same thing” in the sense of being the same font face and font size it was before you syntax-highlighted it.  If you used a line spacing other than 1.0, it will be reset to 1.0.  This is due to a limitation in highlight, which doesn’t accept line-height values as an argument, and thus will always return text with 1.0 spacing.  It’s likely that other fancy adjustments like kerning will also be reset to default, though I didn’t test them all.  I just know that highlight only accepts font name and size as styling parameters, so those were the only ones I could affect.

If you want to set up something similar for HTML, then you need only duplicate the workflow to a different name  —  say, “Syntax Highlight HTML”  —  and then change the value of the AppleScript syntaxType variable from css to html in the new workflow.  That’s all.  Similar steps should be taken to set up a workflow for any other recognized language.

There are a few things to note.

1. If you try to syntax-highlight a block of text that contains multiple font faces or sizes, all of the selected text will be reset to the first face and size, or the script will simply fail to work.  As far as I can tell, preserving the face and size on a per-line (or per-character) basis would be difficult to achieve and probably produce terrible RTF.  As a workaround, highlight each bit of differently-sized or -faced text on its own, and invoke the service.
2. There is no checking to see if the text you highlight matches the type of highlighter to apply to it.  If you try to use “Syntax Highlight CSS” on some HTML, the CSS parser will be used.  So be careful.
3. Picking a new theme can be a cumbersome process, involving a fair amount of trial and error.  It looks like Syntax Highlight has a standalone application that can be used for quick previewing, but I haven’t tried it so I can’t vouch for or against it.
4. I did all this in macOS Sequoia.  I would hope it still works in Tahoe, but if not, let me know in the comments and I’ll add a warning note and a heavy sigh.

There are probably more efficient or more elegant ways to do the individual bits of both scripts, but this works for me, so I figured I’d pass it on to anyone else who’d like to use it.  Improvements, or pointers to solid information that can help me overcome the limitations I mentioned, are always welcome!

• In situ Syntax Highlighting in macOS Applications Like Keynote was published on Wednesday, June 17th, 2026.
• It was assigned to the CSS and Tools categories.
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