[Theme Horse Support Team]

Hi Hkaut,

To change the font family to Trebuchet MS in all my pages and posts you need to create a child theme.

Note: To create a child theme visit this below link
https://codex.wordpress.org/Child_Themes

Also we have replied many times how to create a child theme. You just create a child folder and under it create style.css file and add below code.

/*
Theme Name: Clean Retina Child Theme
Theme URI: https://www.themehorse.com/themes/clean-retina
Author: Theme Horse
Author URI: https://www.themehorse.com
Description: Clean Retina is a Simple, Clean and Responsive Retina Ready WordPress Theme which adapts automatically to your tablets and mobile devices. Along with the elegant design the theme is easily customizable with numerous theme options. Some of the highlighted options are 9 home page layouts(corporate layout also included), 5 layouts for every post, featured slider to highlight your post and page, custom post type to show gallery, 11 social icons, webmaster tools, custom background, custom Header, custom menu, custom favicon, custom CSS, custom webclip icon and many more.  Also supports some of the popular plugins like Breadcrumb NavXT, WP-PageNavi and Contact Form 7. Currently translated in Polish, Romanian, French, Swedish, German, Spanish, Persian and Russian. Get free support on https://www.themehorse.com/support-forum/
Version: 3.0
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: black, gray, white, light, blue, one-column, two-columns, three-columns, left-sidebar, right-sidebar, fluid-layout, custom-menu, custom-background, custom-header, featured-images, full-width-template, theme-options, threaded-comments, translation-ready, sticky-post
Text Domain: cleanretina
Template: clean-retina
*/

@import url("../clean-retina/style.css"); 
body, 
input, 
textarea,
h1, h2, h3, h4, h5, h6,
#site-title,
.slogan,
.entry-title, 
.custom-gallery-title,
#access a,
#access select { 
	font-family: "Trebuchet MS",Tahoma,sans-serif;
}

Thank you!