Brompton: set proper configuration for main-nav font

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

@@ -324,6 +324,7 @@ $config-header: (
 		// Fonts
 		"font": (
 			"family": map-deep-get($config-global, "font", "family", "primary"),
+			"family-css-variable": '--font-headings',
 			"size": map-deep-get($config-global, "font", "size", "sm"),
 			"weight": 900,
 			"line-height": 1,

brompton/style-rtl.css

@@ -2725,7 +2725,7 @@ body:not(.fse-enabled) .main-navigation a {
 
 .footer-navigation .footer-menu a {
 	font-family: "Nunito Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif;
-	font-family: var(--font-base, "Nunito Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif);
+	font-family: var(--font-headings, "Nunito Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif);
 	font-weight: 900;
 	padding: 16px;
 	color: currentColor;

brompton/style.css

@@ -2742,7 +2742,7 @@ body:not(.fse-enabled) .main-navigation a {
 
 .footer-navigation .footer-menu a {
 	font-family: "Nunito Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif;
-	font-family: var(--font-base, "Nunito Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif);
+	font-family: var(--font-headings, "Nunito Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen", "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif);
 	font-weight: 900;
 	padding: 16px;
 	color: currentColor;