_editor.less

.CodeMirror {
    font: 14px @monospaced-fonts, monospace;

    &.cm-s-default {
        border-radius: 3px;
        padding: 0 !important;
    }

    .cm-comment {
        background: inherit !important;
    }
}

.repository.file.editor .tab[data-tab="write"] {
    padding: 0 !important;
}

.repository.file.editor .tab[data-tab="write"] .editor-toolbar {
    border: 0 !important;
}

.repository.file.editor .tab[data-tab="write"] .CodeMirror {
    border-left: 0;
    border-right: 0;
    border-bottom: 0;
}