css: Add new css for rendered markdown table in dark mode.

Add a new dark mode css in app_variable.css for rendered
markdown tables. This rule sets a brighter color for tables
in messages / chat to make it more visible when the user is
using dark theme.

Fixes zulip#29856.

web/styles/app_variables.css

@@ -638,7 +638,8 @@
 
     /* Markdown color */
     --color-background-rendered-markdown-thead: hsl(0deg 0% 0% / 50%);
-
+    --color-border-rendered-markdown-table: hsl(0deg 0% 100% / 20%);
+
     /* Markdown code colors */
     /* Note that Markdown code-link colors are identical
        to light theme, and so are not redeclared here. */

web/styles/dark_theme.css

@@ -784,9 +784,6 @@
     #subscription_overlay .settings-radio-input-parent,
     #settings_page .sidebar-wrapper,
     #settings_page .sidebar-wrapper *,
-    table,
-    table th,
-    table td,
     #recent_view_table table td {
         border-color: hsl(0deg 0% 0% / 20%);
     }