Fix: forward theme text colour to chart scenes and theme the comparison card

Chart scenes are unreadable on any dark theme. BarChart, LineChart,
PieChart and KPIGrid all default `textColor` to #1F2937 (near-black),
but Explainer only ever passed them `backgroundColor` - so on a dark
theme every title, axis label, category label and value renders
dark-on-dark.

ComparisonCard has the mirror-image bug: it *does* receive the theme's
`textColor`, but hardcodes `cardBackgroundColor = "#F3F4F6"`, painting
light text onto a light card.

Forward `textColor` to the four chart components, and give
ComparisonCard `cut.cardBackgroundColor || theme.surfaceColor` so its
surface follows the theme. Adds `cardBackgroundColor?: string` to the
Cut interface for a per-cut override.

Verified by re-rendering a 7-scene explainer on a dark theme: chart
title, y-axis scale, category labels and per-bar values are all legible,
and the comparison card labels now show on a themed surface.

remotion-composer/src/Explainer.tsx

@@ -220,6 +220,7 @@ interface Cut {
   heroSubtitle?: string;
   // Styling overrides
   backgroundColor?: string;
+  cardBackgroundColor?: string; // Inner card surface (comparison); defaults to theme.surfaceColor
   backgroundImage?: string; // AI-generated or stock image rendered behind the component
   backgroundVideo?: string; // Video clip rendered behind the component (takes priority over backgroundImage)
   backgroundVideoStart?: number; // Seek position in seconds for background video (default 0)
@@ -602,6 +603,7 @@ const SceneRenderer: React.FC<{ cut: Cut; theme: ThemeConfig }> = ({ cut, theme
         leftLabel={cut.leftLabel} rightLabel={cut.rightLabel}
         leftValue={cut.leftValue} rightValue={cut.rightValue}
         title={cut.title} backgroundColor={bgColor} textColor={textColor}
+        cardBackgroundColor={cut.cardBackgroundColor || theme.surfaceColor}
       />
     );
   }
@@ -640,6 +642,7 @@ const SceneRenderer: React.FC<{ cut: Cut; theme: ThemeConfig }> = ({ cut, theme
         data={cut.chartData} title={cut.title} colors={cut.chartColors || theme.chartColors}
         animationStyle={(cut.chartAnimation as any) || "grow-up"}
         showGrid={cut.showGrid} showValues={cut.showValues} backgroundColor={bgColor}
+        textColor={textColor}
       />
     );
   }
@@ -650,6 +653,7 @@ const SceneRenderer: React.FC<{ cut: Cut; theme: ThemeConfig }> = ({ cut, theme
         animationStyle={(cut.chartAnimation as any) || "draw"}
         showGrid={cut.showGrid} showMarkers={cut.showMarkers} showLegend={cut.showLegend}
         xLabel={cut.xLabel} yLabel={cut.yLabel} backgroundColor={bgColor}
+        textColor={textColor}
       />
     );
   }
@@ -660,6 +664,7 @@ const SceneRenderer: React.FC<{ cut: Cut; theme: ThemeConfig }> = ({ cut, theme
         animationStyle={(cut.chartAnimation as any) || "expand"}
         donut={cut.donut} centerLabel={cut.centerLabel} centerValue={cut.centerValue}
         showLegend={cut.showLegend} backgroundColor={bgColor}
+        textColor={textColor}
       />
     );
   }
@@ -669,6 +674,7 @@ const SceneRenderer: React.FC<{ cut: Cut; theme: ThemeConfig }> = ({ cut, theme
         metrics={cut.chartData} title={cut.title} columns={cut.columns}
         colors={cut.chartColors || theme.chartColors} animationStyle={(cut.chartAnimation as any) || "count-up"}
         backgroundColor={bgColor}
+        textColor={textColor}
       />
     );
   }