How to change font sizes in your themes

Global font-size changes

The simplest way to change the font size of your entire community would be to override the default value on the HTML element in your theme’s CSS, like this:

html {
  font-size: 16px; /* default is 15px */
}

Because all of our font-size values within <HTML> are defined with em units, increasing the font-size on <HTML> will proportionately increase the font-size of all other elements (ems are relative units).

Discourse also comes with user-selectable text size options that can be changed in each users preferences (/my/preferences/interface), by default these are:

Smaller: 14px
Normal: 15px (default)
Larger: 17px
Largest: 19px

When you change the font-size of <HTML> as demonstrated above, you’re only changing the Normal value. So if you want the user text-size settings to continue working in your theme you should also increase the font-size for the other options. If you wanted to increase the font-size of all options by 1px, that would look like this:

html {
    font-size: 16px; /* default 15px  */
    &.text-size-smaller {
      font-size: 15px; /* default 14px  */
    }
    &.text-size-larger {
      font-size: 18px; /* default 17px  */
    }
    &.text-size-largest {
      font-size: 20px; /* default 19px  */
    }
  }

Changing font-size of individual components

You might not want to increase the global font-size of your community, and just change the font-size of a specific component, like the header or posts. If you’re familiar with CSS, you can target individual elements as expected.

For example, to increase the font-size of all the content within a post you can do this:

.topic-post {
  font-size: 1.2em;
}

If you wanted to change the text size of the post content, but not the usernames and other metadata you need to be a little more specific (right click on an element and use your browser’s inspector if you need to figure out which element to target)…

.topic-post .contents {
  font-size: 1.2em;
}

Note that in the above examples I’m using em units. You can use px values here, but the benefit of ems is that they’re relational.

If you used pixel units in the above examples, those font sizes would stay the same even if a user changed the text size setting in their preferences. A static value like 16px is always 16px. But when you use a value like 1.2em, it acts as a multiplier… so if someone chooses a larger text size in their settings the font-size will always scale up to be 1.2x larger than the base setting.

Utilizing Discourse’s font-scaling variables

In Discourse’s default styles we rely on a set of font scaling variables. You can also use these variables in your themes:

$font-up-6: 2.296em;
$font-up-5: 2em;
$font-up-4: 1.7511em;
$font-up-3: 1.5157em;
$font-up-2: 1.3195em;
$font-up-1: 1.1487em; 
$font-0: 1em;
$font-down-1: 0.8706em; 
$font-down-2: 0.7579em; 
$font-down-3: 0.6599em;
$font-down-4: 0.5745em;
$font-down-5: 0.5em;
$font-down-6: 0.4355em;

This system ensures we’re using a limited set of font sizes that scale based on the global size set on html (and saves you from doing math when nesting em units). If an element is set to $font-up-3, we know that it will be 1.5x larger than $font-0, no matter what the specific px value is.

If you feel a bit lost, it might help to visualize these variables like a ladder. If you have an element with font-size: $font-up-3 and needed a child of that element to be the equivalent to $font-0, you would need to go 3 steps down the ladder to get there (so you’d use $font-down-3). Here it is in action:

.topic-post {
  font-size: $font-up-3; // 3 steps up
  .topic-meta-data {
    font-size: $font-down-3; // 3 steps back down; equivalent to $font-0 (1em)
  }
}
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