Shawburn: set proper configuration for main-nav font

shawburn/sass/_config-child-theme-deep.scss

@@ -322,6 +322,7 @@ $config-header: (
 		// Fonts
 		"font": (
 			"family": map-deep-get($config-global, "font", "family", "secondary"),
+			"family-css-variable": '--font-base',
 			"size": map-deep-get($config-global, "font", "size", "sm"),
 			"weight": 700,
 			"line-height": 1,

shawburn/style-rtl.css

@@ -2540,7 +2540,7 @@ body:not(.fse-enabled) .site-description {
 	color: white;
 	display: block;
 	font-family: "PT Sans", Arial, sans-serif;
-	font-family: var(--font-headings, "PT Sans", Arial, sans-serif);
+	font-family: var(--font-base, "PT Sans", Arial, sans-serif);
 	font-weight: 700;
 	padding: 8px 0;
 }

shawburn/style.css

@@ -2557,7 +2557,7 @@ body:not(.fse-enabled) .site-description {
 	color: white;
 	display: block;
 	font-family: "PT Sans", Arial, sans-serif;
-	font-family: var(--font-headings, "PT Sans", Arial, sans-serif);
+	font-family: var(--font-base, "PT Sans", Arial, sans-serif);
 	font-weight: 700;
 	padding: 8px 0;
 }