fix(ui): make the dark theme's .text-muted visible (#1779)

For instance, the input description was unreadable.

Feel free to tune the color according to the UX rules.

ui/src/styles/layout/root-dark.scss

@@ -24,6 +24,7 @@ html.dark {
     #{--bs-border-color}: $border-color;
     #{--bs-link-color}: $secondary;
     #{--bs-link-color-rgb}: to-rgb($secondary);
+    #{--bs-secondary-color}: var(--bs-gray-700);
     --bs-link-hover-color: #D0CDF9;
     #{--card-bg}: $card-bg;
     #{--input-bg}: $input-bg;