Update themes and plugins to support automatic dark mode

[Discourse]

Previously, all colors in Discourse were stored as SCSS variables. To support automatic dark mode color scheme switching, we have converted these colors in core to custom CSS properties. You can easily see the full list in the inspector now:

Themes and plugins need to switch all the $color SCSS variables used in stylesheets to the --color CSS property equivalents. In most cases, this is a simple find and replace task:

-   background-color: $primary-very-low;
+   background-color: var(--primary-very-low);

But there are some cases where a theme or a plugin is using a more complex variation of a color, for example, when darkening or lightening using SCSS color functions. These cases require a more complex refactoring, and for this we have added the capacity to extend color definitions in themes and plugins.

In plugins

This commit in the discourse-encrypt plugin is a good and simple example of such a refactor. It moves a mix($color1, $color2) SCSS declaration into a separate file and stores it as a CSS custom property. Then the new file is registered as a :color_definitions asset which ensures that the newly declared color property is included in the color definitions stylesheet.

In themes

In themes, you can do the same thing by declaring CSS custom properties in the common/color_definitions.scss stylesheet. You can look at this commit in the graceful theme for an example.

Some additional notes

• when using transparent colors via the rgba($color, 0.5) function, SCSS accepts HEX and RGB colors in the first parameter, whereas CSS custom properties only accept an RGB color. That is why we have introduced the hexToRGB() helper and some properties with the --rgb suffix in the color definitions. An example:

// color_definitions.scss
:root {
  --primary: #{$primary};
  --primary-rgb: #{hexToRGB($primary)};
}

// other stylesheet
.element {
  background-color: rgba(var(--primary-rgb), 0.05);
}

• note that in the snippet above, the SCSS variable is interpolated when passed to a custom property. That is a requirement in SCSS, see Sass: Property Declarations for more details.
• the CSS var() declaration can fallback to a second value if the first one is not available, as in, when writing var(--color1, red), CSS will fallback to the red color if the --color1 property is not found. In plugins, we use the SCSS color variables as fallbacks to ensure compatibility with previous versions of Discourse. So the earlier example, would look like this with a fallback:

-   background-color: $primary-very-low;
+   background-color: var(--primary-very-low, $primary-very-low);

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This document is version controlled - suggest changes on github.

[pfaffman (Jay Pfaffman)]

@pmusaraj

Themes and plugins need to switch all the $color SCSS variables used in stylesheets to the --color CSS property equivalents. I

I’m not very good at this stuff and it’ll take me a while to figure this out myself. . . Does this mean that all themes that referred to colors before are now going to be broken?

[pmusaraj (Penar Musaraj)]

No, not at all. SCSS variables in themes will continue to work for a long time.

But any colors outputted via SCSS variables will stay static, i.e. they cannot be dynamically switched to a new color scheme when a browser goes from normal to dark mode. So those themes/plugins will continue to work, they just won’t be compatible with automatic dark mode switching.

[tomtjes (Thomas Reintjes)]

Thanks for the instructions. Is there a way to also change a background image depending on dark/light mode? (I’ve used the theme switcher component to do that.) Would a CSS class indicating the mode be possible?

[pmusaraj (Penar Musaraj)]

Great question, I tried this and noticed that we didn’t properly support using background images or theme variables in the special color definitions stylesheet. So I made some fixes in core, and you should be able to do this now (make sure you pull the latest core).

So, if you have two images in your theme or theme component, with SCSS vars of $bg-light and $bg-dark respectively, you can add this to your color_definitions.scss stylesheet:

$bg: url(dark-light-choose($bg-light, $bg-dark));

:root {
  --custom-bg: #{$bg};
}

And then you can use var(--custom-bg) in your regular stylesheet.

[riking (Kane York)]

For an image, all you need to do is the vanilla prefers-dark-theme CSS media query.

[pmusaraj (Penar Musaraj)]

@riking

vanilla prefers-dark-theme CSS media query

That won’t work well in all cases, because the media query is not aware of the user’s preferences. Users can disable auto-dark-mode switching, but the media query won’t be aware of that, and it will result in the background meant for the dark color scheme being rendered.

[j127]

Is it possible for Discourse to also add a CSS class to the <body> for the color scheme or color scheme ID? It seems like it would be much easier.

I’m trying to solve a problem during a theme conversion where I need a lot of different CSS rules and variables, and it’s turning into a complex mess in the color_definitions.scss file.

If I could just do something like this in an isolated SCSS file in the theme, it would take 5 minutes to do something that is taking a long time to figure out with color_definitions.scss:

body.dark-palette .some-thing {
  // some styles
}

body.light-palette .some-thing {
  // some styles
}

It’s possible that I’m not understanding the best way to do it, but it seems like many things have to be done for for each variable — setting up one of the CSS variables takes up 17 lines in the editor far away from the one small place where it will be used:

• define a light mode value in a SCSS variable (this can be 6 lines in the editor for something like a linear gradient)
• define a dark mode value in a SCSS variable (another 6 lines)
• define a SCSS variable with dark-light-choose (4 lines due to long variable names)
• assign that last SCSS variable to a CSS variable in :root (1 line)

It also takes time to figure out how to name all four of those things, because it’s global.

[pmusaraj (Penar Musaraj)]

Yeah, you can end up with many lines of code in the color definitions files, particularly for gradients.

It’s still the most appropriate “place” for it in my mind. It’s away from the element where it is used, yes, but it is a convenient single place that handles colors/gradients that switch based on light/dark mode.

An alternative is to use something like this:

@container style(--scheme-type: light){
  body{
    background: red;
  }
}

Our color schemes come with a --scheme-type property that is light for light schemes and dark for dark schemes. Recent browsers support container queries, so this should do what you are trying to accomplish here without adding a class to the body of the page.

[j127]

My CSS/SCSS skills aren’t great. Maybe this is easier for people who are deep into those things.

It was making a mess of the color_definitions.scss file, so I moved it into another file in scss/ so I could import it. I’m not sure how to name it all, so it’s looking like this.

I think it would be easier to do with a body class (or maybe some more examples), but in any case, it’s working for now. I don’t know how container queries work, but I’ll research it later.