AB#671 - Inline @hsl-fi/design-system-core styles instead of leaving them as a passthrough import

sass/_main.scss

@@ -2,7 +2,17 @@
 $body-font-family: $font-family;
 $body-font-weight: $font-weight-medium;
 
-@import '@hsl-fi/design-system-core/css/styles.css';
+// Note: the '.css' extension is intentionally dropped here. Dart Sass only
+// leaves an '@import' as a literal, un-inlined passthrough when the path
+// explicitly ends in '.css'; webpack's splitChunks then treats that
+// passthrough import as its own module and, since it resolves under
+// node_modules/@hsl-fi/, pulls it into the shared 'digitransit-components'
+// chunk which loads *after* each theme's own CSS. That let this file's
+// default '--font-weight-medium'/'--font-weight-book' values win over the
+// theme-specific ones set later in this same file by base/helper-mixins.
+// Importing without the extension makes Sass inline the file's contents
+// here instead, so the theme override below still applies last and wins.
+@import '@hsl-fi/design-system-core/css/styles';
 
 // Apply theme specific values over standard hsl theme
 @import 'design-theme.css';