Syntax highlighting has wrong inline background on darkmode

Summary

Syntax highlighting with a lighter background theme, shows a darker inline background.

Steps to reproduce

1. Set your user to use dark theme
2. Set highlighting theme to solarized light
3. Access any syntax highlight snippet in Issuable descriptions

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Check example in: omnibus-gitlab!5472 (merged) (screenshot below)

Possible fixes