[theme][Markdown] Text of shell code blocks with light theme are rendered in the same color as the background in Markdown files in text editor

[corneliusludmann]

Description

The text of shell code blocks in Markdown files are rendered white (same as background) in light theme in the text editor (NOT the Markdown preview).

Reproduction Steps

Create a Markdown file (e.g. README.md) and add the following content:

Build:
```shell
$ yarn && yarn plugins:update && yarn build
```

In the dark theme is looks like this:

However in the light theme, most of the text is missing (same color as the background):

OS and Theia version:

• Theia next (browser)

[corneliusludmann]

It's also in TypeScript:

(The dot before size is missing in the light theme.)

[akosyakov]

I can reproduce markdown issue, but not with typescript.

[akosyakov]

It happens because shell script is registered after markdown language, so markdown tokenizer does not recognize it as an embedded language and does not apply proper coloring. We need to compute id of an embedded language lazily or something like that.