Update Tachyon dark theme colors

When we aligned the dark theme colors with the heatmap legend
palette, the flamegraph became too gloomy. To improve clarity and
contrast, we should use more muted colors for the heatmap and more
vibrant colors for the flamegraph.

Author: ivonastojanovic

Lib/profiling/sampling/_heatmap_assets/heatmap.css

@@ -5,6 +5,18 @@
    This file extends the shared foundation with heatmap-specific styles.
    ========================================================================== */
 
+/* Heatmap heat colors - using base.css colors with 60% opacity */
+[data-theme="dark"] {
+  --heat-1: rgba(90, 123, 167, 0.60);
+  --heat-2: rgba(106, 148, 168, 0.60);
+  --heat-3: rgba(122, 172, 172, 0.60);
+  --heat-4: rgba(142, 196, 152, 0.60);
+  --heat-5: rgba(168, 216, 136, 0.60);
+  --heat-6: rgba(200, 222, 122, 0.60);
+  --heat-7: rgba(244, 212, 93, 0.60);
+  --heat-8: rgba(255, 122, 69, 0.60);
+}
+
 /* --------------------------------------------------------------------------
    Layout Overrides (Heatmap-specific)
    -------------------------------------------------------------------------- */

Lib/profiling/sampling/_shared_assets/base.css

@@ -124,15 +124,15 @@
 
   --header-gradient: linear-gradient(135deg, #21262d 0%, #30363d 100%);
 
-  /* Dark mode heat palette - muted colors that provide sufficient contrast with light text */
-  --heat-1: rgba(74, 123, 167, 0.35);
-  --heat-2: rgba(90, 159, 168, 0.38);
-  --heat-3: rgba(106, 181, 181, 0.40);
-  --heat-4: rgba(126, 196, 136, 0.42);
-  --heat-5: rgba(160, 216, 120, 0.45);
-  --heat-6: rgba(196, 222, 106, 0.48);
-  --heat-7: rgba(244, 212, 77, 0.50);
-  --heat-8: rgba(255, 107, 53, 0.55);
+  /* Dark mode heat palette - cool to warm gradient for visualization */
+  --heat-1: rgba(90, 123, 167, 1);
+  --heat-2: rgba(106, 148, 168, 1);
+  --heat-3: rgba(122, 172, 172, 1);
+  --heat-4: rgba(142, 196, 152, 1);
+  --heat-5: rgba(168, 216, 136, 1);
+  --heat-6: rgba(200, 222, 122, 1);
+  --heat-7: rgba(244, 212, 93, 1);
+  --heat-8: rgba(255, 122, 69, 1);
 
   /* Code view specific - dark mode */
   --code-bg: #0d1117;