Better Sass Media Queries

```
@media (min-width: $screen-sm-max: ($screen-md-min - 1) !default;
$screen-xs-max: ($screen-sm-min - 1) !default;)
```
would set up for something a little easier to read, such as when making a media query:
```
/* SM */
@media (min-width: $screen-sm-min) and ($screen-sm-max)
```

Same could end up going for CSS variables

No files or tests are named. Review the Sass breakpoint variables and media-query examples represented in the issue, clarify the intended variable syntax and whether CSS variables are included, then verify that the resulting queries are easier to read without changing their ranges.

Written by the indexing model from the issue text.