Migrate to latest polished v3 version

`polished` has been updated to the large 3.0.0 version milestone (1)
that comes with many features in form of new modules, improvements like
a new error system as well as a a roadmap for v4.
The `readableColor` helper now offers the option to set the color(s) it
returns for light or dark colors instead of only returning `white` or
`black` based on the passed colors luminosity. `stripUnit` now offers
the option to return the value and unit as an array, replacing the
functionality of `getValueAndUnit` that'll is now deprecated and will be
removed in v4.
All color modules will now also safely handle the `transparent` keyword
instead of erroring out.
See the release notes for all details and changes.

References:
  (1) https://github.com/styled-components/polished/releases/tag/v3.0.0

GH-137

src/styles/theme/utils/ms.js

@@ -22,6 +22,6 @@ import typography from "../typography";
  * @see https://polished.js.org/docs/#modularscale
  * @since 0.3.0
  */
-const ms = step => modularScale(step, typography.sizes.msBase, typography.sizes.msName);
+const ms = step => modularScale(step, `${typography.sizes.msBase}em`, typography.sizes.msName);
 
 export default ms;