Exec=env GTK_THEME=Adwaita-dark GDK_BACKEND=x11 filezilla
If you can't stand the high-contrast Windows theme, you can at least update FileZilla's icons to look better against a dark background. filezilla dark theme upd
If you downloaded a custom theme, ensure it is placed back into the correct directory. On Windows, this is typically located at C:\Program Files\FileZilla FTP Client\resources\ . Issue 3: Directory Listing Background Stays White filezilla dark theme upd
The icons inside FileZilla (buttons for connect, disconnect, folder structures) can be changed independently using custom icon packs. filezilla dark theme upd
FileZilla Dark Theme Update: How to Enable Dark Mode Across All Operating Systems