style: increase chat text and heading contrast

Boosted assistant message text opacity for better readability
Styled markdown headings with a slightly stronger foreground color

packages/ui/src/components/chat/message/MessageBody.tsx

@@ -2162,7 +2162,7 @@ const AssistantMessageBody = React.memo(({
              ) : null}
               <div>
                  <div
-                     className="message-content-text leading-relaxed overflow-hidden text-foreground/80 [&_p:last-child]:mb-0 [&_ul:last-child]:mb-0 [&_ol:last-child]:mb-0"
+                     className="message-content-text leading-relaxed overflow-hidden text-foreground/90 [&_p:last-child]:mb-0 [&_ul:last-child]:mb-0 [&_ol:last-child]:mb-0"
                  >
                     {renderedParts}
                     {showErrorMessage && (

packages/ui/src/index.css

@@ -1134,6 +1134,7 @@ html:not(.dark) .chat-scroll {
 .markdown-content h5,
 .markdown-content h6 {
   margin: 1.25rem 0 0.5rem;
+  color: color-mix(in srgb, var(--foreground) 95%, transparent);
   font-weight: 600;
   line-height: 1.3;
 }