Fixes #3843

Cause

When renaming an icon on the desktop with dark GTK themes (such as Adwaita-dark), the filename appears as near-white text on a white gradient background with an invisible black text caret.

This occurs due to a CSS cascade specificity conflict between the two shipped stylesheets:

1. nemo-style-application.css defines:

   .nemo-desktop.view .entry,
   .nemo-desktop.view .entry:active,
   .nemo-desktop.view .entry:focus,
   .nemo-desktop.view .entry:backdrop {
       color: #000000;
       caret-color: #000000;
       background-image: -gtk-gradient(linear, ...);
   }

2. nemo-style-fallback-mandatory.css defines:

   .nemo-window .nemo-window-pane widget.entry {
       color: @theme_fg_color;
       border-color: @theme_selected_bg_color;
       background-color: @theme_bg_color;
   }

Because the desktop window carries both .nemo-window/.nemo-window-pane and .nemo-desktop, the mandatory rule matches the desktop rename entry (widget.entry) with higher specificity (0,3,1 vs 0,3,0).

The mandatory rule overrides color (to @theme_fg_color, light in dark theme) and background-color (to @theme_bg_color, dark in dark theme), but omits background-image and caret-color. As a result:

• The hardcoded white gradient from nemo-style-application.css is not overridden and is painted over @theme_bg_color, producing near-white text on a white gradient.
• caret-color remains hardcoded to #000000, making the caret invisible against the dark background.

Solution

In gresources/nemo-style-fallback-mandatory.css:

• Added background-image: none; and caret-color: @theme_fg_color; to .nemo-window .nemo-window-pane widget.entry.
• Added background-image: none; to .nemo-window .nemo-window-pane widget.entry:selected for consistency across entry selection states.

Verification

• Tested stylesheet parsing and GtkStyleContext resolution using GTK 3 (gi.repository.Gtk):
  • Verified EelEditableLabel widget path resolution (window.nemo-window.nemo-desktop-window > box.nemo-window-pane > box.nemo-desktop.view > widget.entry).
  • Confirmed under dark theme palette that background-image resolves to None in both normal and selected states, caret-color resolves to @theme_fg_color, and text/background colors properly adhere to theme tokens (@theme_fg_color / @theme_bg_color).
  • Validated that fallback mandatory stylesheet loads cleanly across both @theme_ and @ variable naming conventions handled by src/nemo-application.c.