DO NOT MERGE: Add K6 overrides to light theme.

[cjcenizal]

DO NOT MERGE.

☠️ ☠️ ☠️

This branch is intended to be consumed by Kibana 6.x. By providing a K6 theme over the K7 components, we're hoping to enable Kibana engineers to consume this framework today, while retaining a consistent look-and-feel in the Kibana UI.

Guidelines

1. Only override SCSS variables.
2. If necessary, abstract parts of KUI behind variables and mixins to enable No Elasticsearch assets SVG #1.
3. Avoid overriding selectors and styles directly as much as possible. Only do this as a last resort.

K6 theme

The K6 theme consists of:

1. Changing colors to match the palette.
2. Changing font-size to match.
3. Changing some spacing to match, which generally just means making spacing tighter.

[snide]

Closing in place of #191