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<?php
function hostvn_add_editor_styles() {
    add_editor_style( CSS_SRC . 'editor.css' );
    add_editor_style('https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.5.0/css/font-awesome.min.css');
}
add_action( 'admin_init', 'hostvn_add_editor_styles' );

<?php
if ( ! function_exists( 'wpex_mce_google_fonts_styles' ) ) {
    function wpex_mce_google_fonts_styles() {
        $font_url = 'https://fonts.googleapis.com/css?family=Open+Sans:300,400,500,600,700&subset=latin,vietnamese,latin-ext';
        add_editor_style( str_replace( ',', '%2C', $font_url ) );
    }
}
add_action( 'init', 'wpex_mce_google_fonts_styles' );

<?php
if ( ! function_exists( 'wpex_mce_text_sizes' ) ) {
    function wpex_mce_text_sizes( $initArray ){
        $initArray['fontsize_formats'] = "9px 10px 12px 13px 14px 16px 18px 21px 24px 28px 32px 36px";
        return $initArray;
    }
}
add_filter( 'tiny_mce_before_init', 'wpex_mce_text_sizes' );

<?php
if ( ! function_exists( 'wpex_mce_google_fonts_array' ) ) {
    function wpex_mce_google_fonts_array( $initArray ) {
        $initArray['font_formats'] = 'Open Sans=Open Sans,Helvetice,sans-serif;Lato=Lato;Andale Mono=andale mono,times;Arial=arial,helvetica,sans-serif;Arial Black=arial black,avant garde;Book Antiqua=book antiqua,palatino;Comic Sans MS=comic sans ms,sans-serif;Courier New=courier new,courier;Georgia=georgia,palatino;Helvetica=helvetica;Impact=impact,chicago;Symbol=symbol;Tahoma=tahoma,arial,helvetica,sans-serif;Terminal=terminal,monaco;Times New Roman=times new roman,times;Trebuchet MS=trebuchet ms,geneva;Verdana=verdana,geneva;Webdings=webdings;Wingdings=wingdings,zapf dingbats';
        return $initArray;
    }
}
add_filter( 'tiny_mce_before_init', 'wpex_mce_google_fonts_array' );