theme_bootstrap/less/moodle/reports.less

/* reports.less */

// The home for small tweaks to reports that don't require
// changes drastic enough to pull in the full module css
// and replace it completely.

#page-report-participation-index .participationselectform div label { // Using 'div' here to override the report styles.css
    display: inline;
    margin: 0 5px 0 0;
}