Remove text-shadow from code blocks for all themes

Several gitlab themes make use of text-shadow's throughout, and that's okay. But unfortunately in some cases, such as code embedded in an issue, the text-shadow gets inherited by the syntax highlight code block. This creates an undesirable effect where code blocks look fuzzy and lose clarity.

I have added a catch-all rule to common.scss that will ensure that themes can use text-shadows without causing blurry code.