Fixes lobby backgrounds. (#7557)

Content.Server/GameTicking/Prototypes/LobbyBackgroundPrototype.cs

@@ -0,0 +1,22 @@
+using JetBrains.Annotations;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Utility;
+
+namespace Content.Server.GameTicking.Prototypes;
+
+/// <summary>
+/// Prototype for a lobby background the game can choose.
+/// </summary>
+[Prototype("lobbyBackground")]
+public sealed class LobbyBackgroundPrototype : IPrototype
+{
+    /// <inheritdoc/>
+    [IdDataField]
+    public string ID { get; set; } = default!;
+
+    /// <summary>
+    /// The sprite to use as the background. This should ideally be 1920x1080.
+    /// </summary>
+    [DataField("background", required: true)]
+    public ResourcePath Background = default!;
+}