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] .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] .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]:active, atom-text-editor[mini].active::shadow => atom-text-editor[mini]:active, atom-text-editor[mini].active.editor
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[mini].is-focused .selection .region, atom-text-editor[mini].is-focused::shadow .selection .region => atom-text-editor[mini].is-focused .selection .region, atom-text-editor[mini].is-focused.editor .selection .region
atom-text-editor, atom-text-editor::shadow => atom-text-editor, atom-text-editor.editor
atom-text-editor .wrap-guide, atom-text-editor::shadow .wrap-guide => atom-text-editor .wrap-guide, atom-text-editor.editor .wrap-guide
atom-text-editor .cursor, atom-text-editor::shadow .cursor => atom-text-editor .cursor, atom-text-editor.editor .cursor
atom-text-editor .line.cursor-line, atom-text-editor::shadow .line.cursor-line => atom-text-editor .line.cursor-line, atom-text-editor.editor .line.cursor-line
atom-text-editor .comment, atom-text-editor::shadow .comment => atom-text-editor .comment, atom-text-editor.editor .syntax--comment
atom-text-editor .highlight.selection > .region, atom-text-editor::shadow .highlight.selection > .region => atom-text-editor .highlight.selection > .region, atom-text-editor.editor .highlight.selection > .region
atom-text-editor .highlight.bracket-matcher .region, atom-text-editor::shadow .highlight.bracket-matcher .region => atom-text-editor .highlight.bracket-matcher .region, atom-text-editor.editor .highlight.bracket-matcher .region
atom-text-editor .gutter, atom-text-editor::shadow .gutter => atom-text-editor .gutter, atom-text-editor.editor .gutter
atom-text-editor .gutter[gutter-name="linter"], atom-text-editor::shadow .gutter[gutter-name="linter"] => atom-text-editor .gutter[gutter-name="linter"], atom-text-editor.editor .gutter[gutter-name="linter"]
atom-text-editor .gutter[gutter-name="linter"] .custom-decorations, atom-text-editor::shadow .gutter[gutter-name="linter"] .custom-decorations => atom-text-editor .gutter[gutter-name="linter"] .custom-decorations, atom-text-editor.editor .gutter[gutter-name="linter"] .custom-decorations
atom-text-editor .gutter[gutter-name="linter"] .custom-decorations .linter-row, atom-text-editor::shadow .gutter[gutter-name="linter"] .custom-decorations .linter-row => atom-text-editor .gutter[gutter-name="linter"] .custom-decorations .linter-row, atom-text-editor.editor .gutter[gutter-name="linter"] .custom-decorations .linter-row
atom-text-editor .gutter[gutter-name="linter"] .custom-decorations .linter-row .linter-gutter, atom-text-editor::shadow .gutter[gutter-name="linter"] .custom-decorations .linter-row .linter-gutter => atom-text-editor .gutter[gutter-name="linter"] .custom-decorations .linter-row .linter-gutter, atom-text-editor.editor .gutter[gutter-name="linter"] .custom-decorations .linter-row .linter-gutter
atom-text-editor .gutter .line-numbers, atom-text-editor::shadow .gutter .line-numbers => atom-text-editor .gutter .line-numbers, atom-text-editor.editor .gutter .line-numbers
atom-text-editor .gutter .line-number, atom-text-editor::shadow .gutter .line-number => atom-text-editor .gutter .line-number, atom-text-editor.editor .gutter .line-number
atom-text-editor .gutter .line-number.cursor-line, atom-text-editor::shadow .gutter .line-number.cursor-line => atom-text-editor .gutter .line-number.cursor-line, atom-text-editor.editor .gutter .line-number.cursor-line
atom-text-editor .gutter .line-number.git-line-modified, atom-text-editor::shadow .gutter .line-number.git-line-modified => atom-text-editor .gutter .line-number.git-line-modified, atom-text-editor.editor .gutter .line-number.git-line-modified
atom-text-editor .gutter .line-number.git-line-modified.cursor-line, atom-text-editor::shadow .gutter .line-number.git-line-modified.cursor-line => atom-text-editor .gutter .line-number.git-line-modified.cursor-line, atom-text-editor.editor .gutter .line-number.git-line-modified.cursor-line
atom-text-editor .gutter .line-number.git-line-added, atom-text-editor::shadow .gutter .line-number.git-line-added => atom-text-editor .gutter .line-number.git-line-added, atom-text-editor.editor .gutter .line-number.git-line-added
atom-text-editor .gutter .line-number.git-line-added.cursor-line, atom-text-editor::shadow .gutter .line-number.git-line-added.cursor-line => atom-text-editor .gutter .line-number.git-line-added.cursor-line, atom-text-editor.editor .gutter .line-number.git-line-added.cursor-line
atom-text-editor .gutter .line-number.git-line-removed, atom-text-editor::shadow .gutter .line-number.git-line-removed => atom-text-editor .gutter .line-number.git-line-removed, atom-text-editor.editor .gutter .line-number.git-line-removed
atom-text-editor .gutter .line-number.git-line-removed.cursor-line, atom-text-editor::shadow .gutter .line-number.git-line-removed.cursor-line => atom-text-editor .gutter .line-number.git-line-removed.cursor-line, atom-text-editor.editor .gutter .line-number.git-line-removed.cursor-line
atom-text-editor #linter-inline, atom-text-editor::shadow #linter-inline => atom-text-editor #linter-inline, atom-text-editor.editor #linter-inline

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.