Dalston: set proper configuration for main-nav font

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

@@ -323,6 +323,7 @@ $config-header: (
 		// Fonts
 		"font": (
 			"family": map-deep-get($config-global, "font", "family", "secondary"),
+			"family-css-variable": '--font-base',
 			"size": map-deep-get($config-global, "font", "size", "base"),
 			"weight": 600,
 			"line-height": 1,

dalston/style-rtl.css

@@ -2537,7 +2537,7 @@ body:not(.fse-enabled) .site-description {
 	color: #0073AA;
 	display: block;
 	font-family: "Crimson Text", "Baskerville Old Face", Garamond, "Times New Roman", serif;
-	font-family: var(--font-headings, "Crimson Text", "Baskerville Old Face", Garamond, "Times New Roman", serif);
+	font-family: var(--font-base, "Crimson Text", "Baskerville Old Face", Garamond, "Times New Roman", serif);
 	font-weight: 600;
 	padding: 8px 0;
 }

dalston/style.css

@@ -2554,7 +2554,7 @@ body:not(.fse-enabled) .site-description {
 	color: #0073AA;
 	display: block;
 	font-family: "Crimson Text", "Baskerville Old Face", Garamond, "Times New Roman", serif;
-	font-family: var(--font-headings, "Crimson Text", "Baskerville Old Face", Garamond, "Times New Roman", serif);
+	font-family: var(--font-base, "Crimson Text", "Baskerville Old Face", Garamond, "Times New Roman", serif);
 	font-weight: 600;
 	padding: 8px 0;
 }