Stow: set proper configuration for main-nav font

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

@@ -324,6 +324,7 @@ $config-header: (
 		// Fonts
 		"font": (
 			"family": map-deep-get($config-global, "font", "family", "secondary"),
+			"family-css-variable": '--font-base',
 			"size": map-deep-get($config-global, "font", "size", "md"),
 			"weight": 700,
 			"line-height": 1,

stow/style-rtl.css

@@ -2541,7 +2541,7 @@ body:not(.fse-enabled) .site-description {
 	color: #404040;
 	display: block;
 	font-family: "Source Sans Pro", Arial, sans-serif;
-	font-family: var(--font-headings, "Source Sans Pro", Arial, sans-serif);
+	font-family: var(--font-base, "Source Sans Pro", Arial, sans-serif);
 	font-weight: 700;
 	padding: 8px 0;
 }

stow/style.css

@@ -2558,7 +2558,7 @@ body:not(.fse-enabled) .site-description {
 	color: #404040;
 	display: block;
 	font-family: "Source Sans Pro", Arial, sans-serif;
-	font-family: var(--font-headings, "Source Sans Pro", Arial, sans-serif);
+	font-family: var(--font-base, "Source Sans Pro", Arial, sans-serif);
 	font-weight: 700;
 	padding: 8px 0;
 }