This stylelint-scss rule is useful because we were inconsistent when
using calc(); sometimes we added interpolation to Sass variables, and
sometimes we didn't. The reason why we originally added interpolation
was that it was necessary until we migrated to Dart Sass in commit
d54971e53. Since then, we can omit the interpolation, which is also what
the Sass documentation recommends [1].
[1] https://sass-lang.com/documentation/values/calculations/
We hadn't added this rule before because there was no such rule in
scss-lint. Instead, we were following it without a linter, and so we
unintentionally broke it sometimes.
But now we're using Stylelint, so we can add the rule and let the linter
check we're still following it.
The division operator `/` from Sass is deprecated because `/` is used in
CSS for uses other than dividing numbers. That's why we were getting
many warnings like:
```
Deprecation Warning: Using / for division outside of calc() is
deprecated and will be removed in Dart Sass 2.0.0.
Recommendation: math.div($line-height, 2) or calc($line-height / 2)
More info and automated migrator: https://sass-lang.com/d/slash-div
margin-top: $line-height / 2;
```
Since using math.div makes the code harder to read and `calc` is
universally supported by all browsers (although the implementation in
Internet Explorer doesn't work in certain cases), we're using `calc`
when assigning the value to a CSS property.
However, we're also using divisions when assigning Sass variables, and
in those cases using `calc` is trickier because sometimes these
variables are used in other operations. We'll handle these cases in the
next commit.
We're using `background: #fff` and `background: $white` in many places.
Sometimes we mean "use the same background as the body", which means if
we change the body background so it's, let's say, dark, we'll also have
to change all these places.
So now we're using `$body-background` in more places, so changing the
general background color is easier.
There are still some places where we use `#fff` or `$white`. Sometimes
it's hard to tell whether the intention is "use a white background here"
or "use the same background as the body here". When in doubt, I've left
it the way it was.
Just for testing purposes, I've tested locally how things would look
like if we added this code to `_consul_custom_overrides.scss`:
```
$body-background: #fea;
$card-background: $body-background;
$tab-background: $body-background;
$tab-content-background: $body-background;
$table-background: $body-background;
```
Or:
```
$body-background: #333;
$text: #fcfcfc;
$body-font-color: $text;
$card-background: $body-background;
$tab-background: $body-background;
$tab-content-background: $body-background;
$table-background: $body-background;
```
Testing shows we've still got a long way to go to make it easy to add
custom color themes, since there are many custom colors in the code.
Hopefully these changes bring us one step closer.
Internet Explorer 9 was released eight years ago. Besides that, we don't
really support IE8 anyway, since we show a popup to IE8 users saying
we don't support it, we haven't maintained the IE8-specific CSS file for
years, and we don't test our JavaScript against IE8.
