Default colors should adapt to macOS Mojave dark mode

## What did you do?

Rendered a chart under dark mode in Mojave.

## What did you expect to happen?

Labels, grids, etc, to show in white color.

## What happened instead?

Labels, grids, etc, show in black.

## Charts Environment

**Charts version/Branch/Commit Number:master**
**Xcode version:10.1**
**Swift version:4.2**
**Platform(s) running Charts:macOS**
**macOS version running Xcode:10.14.1 Mojave**

Reproduce a chart on macOS Mojave with dark mode enabled and inspect the default color configuration used for labels and grids. The issue is done when those elements adapt to dark mode and render in white rather than black.

Written by the indexing model from the issue text.