Implementing admonitions in Hugo without theme support

The issue

I’m a huge fan of admonitions (like this one below), but couldn’t find a proper way to implement them on top of a hugo theme (e.g. the one I’m using now) without touching it.

Hi! I’m an admonition in Hugo in a theme that does not implement them (nor allows the import of CSS)!


My approach is based on Hugo shortcodes.
Unfortunately, searching for info the only blog page I found was broken (and there was no working archived version), so here is what I did:

I created a new shortcode (layouts/shortcodes/admonition.html):

{{ if not (.Page.Scratch.Get "hasAdmonitions")}}
{{ .Page.Scratch.Set "hasAdmonitions" "True" }}
    {{ $defaultBgColor := "#00bcf6"}}
        border-radius: 5px;
        padding: 0px;
        border-left: 5px solid {{$defaultBgColor}};
        box-shadow: 0 0 .5rem .2rem #00000025;
        background-color: {{$defaultBgColor}};
        border-top-right-radius: 5px;
    .admonition-title {
        font-weight: bolder;
        font-size: large;
        backdrop-filter: grayscale(50%) brightness(150%);
        -webkit-backdrop-filter: grayscale(50%) brightness(150%);
        padding: 5px 0 5px 30px;
        border-top-right-radius: 5px;
    @media (prefers-color-scheme: dark) {
        .admonition-title {
            backdrop-filter: grayscale(40%) brightness(40%);
            -webkit-backdrop-filter: grayscale(40%) brightness(40%);
        padding: 10px 0 10px 15px
{{ end }}

<div class="admonition" {{if .Get "bg-color"}}style="border-left: 5px solid {{ .Get "bg-color" }};"{{end}}>
    <div class="admonition-title-container" {{if .Get "bg-color"}}style="background-color: {{ .Get "bg-color" }};"{{end}}>
        <div class="admonition-title">
            {{ .Get "title" | default "Tip" }}
        <div class="admonition-content">{{ printf "%s" .Inner | markdownify }}</div>

Of course this is totally not inspired by Material for mkdocs

How to use

The green admonitions above is created with this snippet:

{{<admonition title="Admonition" bg-color="#00ee88">}}
Hi! I'm an admonition in Hugo in a theme that does not implement them (nor custom CSS)!

and the bg-color is totally optional.

If you can read the CSS code you can see the “trickery” I did to have a lighter background.
Not really proud of that to be honest, but it kinda works, so that’s good enough. Just remember to use  BRIGHT  colors, or the filter will hide the background color in the title section

How it works

Ignoring the CSS for the lighter code (which is just a backdrop-filter), the way this works is pretty easy: we just format some text in some HTML.
(What I believe) is the most interesting part here is how the CSS is being managed

Keep in mind that:

  1. The template I use does not implements admonitions
  2. The template I use does not allow for using user-provided CSS stylesheets
  3. I did not want to tamper with the theme in any way

Since I’m not tampering with the theme, I won’t put the CSS in the head section of the page. This means I need to put the stylesheet in the HTML page.
I had two possible routes:

  • Use the `<CSS CODE>`| resources.FromString "css/admonitions.css" | resources.ToCSS}} snippet to create a dummy CSS file to reference
  • Paste the stylesheet directly in the HTML

Since the stylesheet is so tiny, I just decided to drop it in the HTML body.

This now poses a new problem: I don’t even want to bloat the HTML page with redundant CSS for each admonition: I just want to define the CSS once.
This is the reason I used the page scratch (.Page.Scratch.Get), which is a “store” where I can define and use page-scoped variables.

I proceeded to create a check for a variable that I instantiate only once I’ve dropped the CSS in the page in the first place.

There you have it, enjoy!