The WordPress tinymce stripe out some tag inlcuding style tag. You can add support for style tag.

Add the following code in functions.php

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add_filter('tiny_mce_before_init', 'prefix_filter_tiny_mce_before_init');

function prefix_filter_tiny_mce_before_init( $options ) {

    if ( ! isset( $options['extended_valid_elements'] ) ) {

        $options['extended_valid_elements'] = 'style';

    } else {

        $options['extended_valid_elements'] .= ',style';

    }

    if ( ! isset( $options['valid_children'] ) ) {

        $options['valid_children'] = '+body[style]';

    } else {

        $options['valid_children'] .= ',+body[style]';

    }

    if ( ! isset( $options['custom_elements'] ) ) {

        $options['custom_elements'] = 'style';

    } else {

        $options['custom_elements'] .= ',style';

    }

    return $options;

}

Source :- https://stackoverflow.com/questions/40825127/wordpress-stripping-style-tags-from-document