In genesis-ui\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[mini], atom-text-editor[mini]::shadow => atom-text-editor[mini], atom-text-editor[mini].editor

• atom-text-editor[mini] .placeholder-text, atom-text-editor[mini]::shadow .placeholder-text => atom-text-editor[mini] .placeholder-text, atom-text-editor[mini].editor .placeholder-text

• atom-text-editor[mini] .cursor, atom-text-editor[mini]::shadow .cursor => atom-text-editor[mini] .cursor, atom-text-editor[mini].editor .cursor

• atom-text-editor[mini] .selection .region, atom-text-editor[mini]::shadow .selection .region => atom-text-editor[mini] .selection .region, atom-text-editor[mini].editor .selection .region

• atom-text-editor[mini].is-focused, atom-text-editor[mini].is-focused::shadow => atom-text-editor[mini].is-focused, atom-text-editor[mini].is-focused.editor

• atom-text-editor .gutter.drop-shadow, atom-text-editor::shadow .gutter.drop-shadow => atom-text-editor .gutter.drop-shadow, atom-text-editor.editor .gutter.syntax--drop-shadow

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.