fix: polish chat related files UI (#241751)

* fix: adopt dashed border for chat related files

* fix: decrease + button size for chat related files

Author: joyceerhl

src/vs/workbench/contrib/chat/browser/media/chat.css

@@ -720,6 +720,7 @@ have to be updated for changes to the rules above, or to support more deeply nes
 	border-radius: 4px;
 	height: 18px;
 	border: 1px solid var(--vscode-input-border);
+	border-style: dashed;
 	align-items: center;
 	overflow: hidden;
 	padding: 0 0 0 2px;
@@ -730,7 +731,8 @@ have to be updated for changes to the rules above, or to support more deeply nes
 	color: var(--vscode-descriptionForeground);
 	padding-right: 4px;
 	padding-left: 4px;
-	padding-top: 3px;
+	padding-top: 5px;
+	font-size: 14px; /* The + codicon is large, make it look more like the x codicon */
 	height: 100%;
 }