Thick lines are not rendered for Mermaid diagram

Description

Meermaid isn't rendered correctly in Gitlab.

Thick arrows don't work

This code:

graph TD
Arrows -->|thin| works
Arrows ==>|thick| fails

should be rendered like this:

Instead it is rendered like this in Gitlab:

graph TD
Arrows -->|thin| works
Arrows ==>|thick| fails

Here is a static image of this gitlab-version of the image instead of a generated one:

As you can see, the thick arrow in gitlab is not thick.

You can see a correct version of the image in the mermaid-live-editor

Note also how the colors are changed.

A reference about how thick arrows should look like in mermaid can be found in the official docs: https://mermaidjs.github.io/

I am using Firefox version 65.0 on macOS (64 bit)