In snik-syntax/index.less:

Starting from Atom v1.13.0, the contents of atom-text-editor elements are no longer encapsulated within a shadow DOM boundary. This means you should stop using :host and ::shadow pseudo-selectors, and prepend all your syntax selectors with syntax--. To prevent breakage with existing style sheets, Atom will automatically upgrade the following selectors:

• atom-text-editor .search-results .marker .region => atom-text-editor .search-results .syntax--marker .region

• atom-text-editor .search-results .marker.current-result .region => atom-text-editor .search-results .syntax--marker.current-result .region

Automatic translation of selectors will be removed in a few release cycles to minimize startup time. Please, make sure to upgrade the above selectors as soon as possible.

• Indentation guides
• Tabs/Spaces
• EOL