fix: #57282: icon color mismatch on download button in public share

The download button icon on the public share page always appeared
in the opposite color of the button text. The root cause was the
wrong CSS variable for the filter applied to the icon.

Background-image icons are dark (black) by default. In light mode,
the icon must be inverted to white when the primary color is dark,
which requires --primary-invert-if-dark. The code was incorrectly
using --primary-invert-if-bright, inverting in the wrong direction.

In dark mode, icons.css swaps the icon variables so that
--icon-download-dark resolves to the white SVG. The filter logic
must be reversed: --primary-invert-if-bright is needed to invert
the white icon to black when the primary color is bright.

Fix by using --primary-invert-if-dark in light mode and
--primary-invert-if-bright in dark mode, handling both the
prefers-color-scheme media query and the Nextcloud data-themes
attribute for explicit theme selection.

Signed-off-by: Rodrigo Mendes Correia <rodrigo.mendes.correia@tecnico.ulisboa.pt>

Author: rodrigocorreiaist

core/src/views/PublicPageMenu.vue

@@ -135,7 +135,23 @@ function openDialogIfNeeded() {
 	}
 
 	&__primary-icon {
-		filter: var(--primary-invert-if-bright);
+		// Light mode: icon is black by default, invert to white when primary is dark
+		filter: var(--primary-invert-if-dark);
+
+		// Dark mode: icon is white (swapped in icons.css), invert to black when primary is bright
+		@media (prefers-color-scheme: dark) {
+			filter: var(--primary-invert-if-bright);
+		}
 	}
 }
+
+// Dark theme via Nextcloud setting (data-themes attribute, not media query)
+:global([data-themes*=dark]) .public-page-menu__primary-icon {
+	filter: var(--primary-invert-if-bright);
+}
+
+// Light theme explicitly set (overrides dark media query if system is dark but user chose light)
+:global([data-themes*=light]) .public-page-menu__primary-icon {
+	filter: var(--primary-invert-if-dark);
+}
 </style>