LibWeb: Implement css gradient-interpolation-method

Author: https://github.com/Gingeh
Commit: 31853c13d3
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3638
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/LucasChollet

Libraries/LibWeb/CSS/StyleValues/CSSLightDark.h

@@ -26,7 +26,7 @@ public:
 
 private:
     CSSLightDark(ValueComparingNonnullRefPtr<CSSStyleValue> light, ValueComparingNonnullRefPtr<CSSStyleValue> dark)
-        : CSSColorValue(CSSColorValue::ColorType::LightDark)
+        : CSSColorValue(CSSColorValue::ColorType::LightDark, ColorSyntax::Modern)
         , m_properties { .light = move(light), .dark = move(dark) }
     {
     }