Using the placeholder selector, we weren't overwriting the rules in the
mixin called with `@include` in some cases because in the generated CSS
the placeholder selector appeared before the code generated by the calls
to `@include`.
Since we were defining the selection to have the same text color and
background color as the element they were in, it resulted in the
selection being invisible.
It wasn't that noticeable because we were using this color combination
mainly in links and buttons, and selecting their text is not as common.
But we plan to use the `$brand` color on budget headers as well, and
this issue is more obvious there.
Browsers like Chrome weren't that affected because they automatically
make the selection semi-transparent and so the selected text still had a
slightly different color. In order to prevent this effect when the
selection is white, we're using a 0.99 opacity (in this case Chrome
ignores numbers higher that 0.998).
Setting the color to `$white` or `#fff` while setting the background to
`$brand` is a pattern we were using in many places. Since we're going to
edit it in order to fix the `::selection` behavior, we're defining the
pattern in one place.
One file was OK when we only had a couple of mixins, but recently we've
been adding quite a few.
We can now avoid a SCSS Lint warning by excluding just the file with the
affected mixin.
