style(web): soften streamdown code block visual weight

Author: Cai-Tang-www

web/src/index.css

@@ -115,8 +115,8 @@
   --user-bubble:    #e7ebf1;
   --user-bubble-text: #1f2937;
 
-  --code-bg:        #f0f0f2;
-  --code-header:    #e8e8ed;
+  --code-bg:        #f7f8fb;
+  --code-header:    #f3f5f8;
   --diff-add-text:  #166534;
   --diff-add-bg:    rgba(34, 197, 94, 0.18);
   --diff-del-text:  #b42318;
@@ -728,7 +728,7 @@ html, body, #root {
 .markdown-streamdown [data-streamdown="code-block"] {
   margin: 0.8em 0;
   border-radius: var(--radius-md);
-  border: 1px solid var(--border-primary);
+  border: 1px solid color-mix(in srgb, var(--border-primary) 68%, transparent);
   background: var(--code-bg);
   position: relative;
   overflow: hidden;
@@ -742,8 +742,8 @@ html, body, #root {
   gap: 4px;
   padding: 2px;
   border-radius: var(--radius-sm);
-  border: 1px solid var(--border-primary);
-  background: color-mix(in srgb, var(--bg-elevated) 88%, transparent);
+  border: 1px solid color-mix(in srgb, var(--border-primary) 62%, transparent);
+  background: color-mix(in srgb, var(--bg-elevated) 78%, transparent);
   z-index: 2;
 }
 .markdown-streamdown [data-streamdown="code-block-copy-button"] {
@@ -768,7 +768,7 @@ html, body, #root {
   align-items: center;
   min-height: 30px;
   padding: 6px 40px 6px 12px;
-  border-bottom: 1px solid var(--border-primary);
+  border-bottom: 1px solid color-mix(in srgb, var(--border-primary) 65%, transparent);
   background: var(--code-header);
 }
 .markdown-streamdown [data-streamdown="code-block-header"] span {