Add checkered colors for defended borders (#691)

## Description:

The borders near a defense post are currently colored differently, but
too discreetly for the user.

I suggest coloring them with a checkerboard pattern.


![image](https://github.com/user-attachments/assets/8f414976-bada-4793-812a-10f28da911f5)

![image](https://github.com/user-attachments/assets/81261148-88e3-4498-9f29-27a6a201b6c5)


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

src/core/configuration/Config.ts

@@ -143,9 +143,8 @@ export interface Theme {
   territoryColor(playerInfo: PlayerView): Colord;
   specialBuildingColor(playerInfo: PlayerView): Colord;
   borderColor(playerInfo: PlayerView): Colord;
-  defendedBorderColor(playerInfo: PlayerView): Colord;
+  defendedBorderColors(playerInfo: PlayerView): { light: Colord; dark: Colord };
   focusedBorderColor(): Colord;
-  focusedDefendedBorderColor(): Colord;
   terrainColor(gm: GameMap, tile: TileRef): Colord;
   backgroundColor(): Colord;
   falloutColor(): Colord;

src/core/configuration/PastelTheme.ts

@@ -98,21 +98,17 @@ export const pastelTheme = new (class implements Theme {
       b: Math.max(tc.b - 40, 0),
     });
   }
-  defendedBorderColor(player: PlayerView): Colord {
-    const bc = this.borderColor(player).rgba;
-    return colord({
-      r: Math.max(bc.r - 40, 0),
-      g: Math.max(bc.g - 40, 0),
-      b: Math.max(bc.b - 40, 0),
-    });
+
+  defendedBorderColors(player: PlayerView): { light: Colord; dark: Colord } {
+    return {
+      light: this.territoryColor(player).darken(0.2),
+      dark: this.territoryColor(player).darken(0.4),
+    };
   }
 
   focusedBorderColor(): Colord {
     return colord({ r: 230, g: 230, b: 230 });
   }
-  focusedDefendedBorderColor(): Colord {
-    return colord({ r: 200, g: 200, b: 200 });
-  }
 
   terrainColor(gm: GameMap, tile: TileRef): Colord {
     const mag = gm.magnitude(tile);

src/core/configuration/PastelThemeDark.ts

@@ -98,21 +98,17 @@ export const pastelThemeDark = new (class implements Theme {
       b: Math.max(tc.b - 40, 0),
     });
   }
-  defendedBorderColor(player: PlayerView): Colord {
-    const bc = this.borderColor(player).rgba;
-    return colord({
-      r: Math.max(bc.r - 40, 0),
-      g: Math.max(bc.g - 40, 0),
-      b: Math.max(bc.b - 40, 0),
-    });
+
+  defendedBorderColors(player: PlayerView): { light: Colord; dark: Colord } {
+    return {
+      light: this.territoryColor(player).darken(0.2),
+      dark: this.territoryColor(player).darken(0.4),
+    };
   }
 
   focusedBorderColor(): Colord {
     return colord({ r: 255, g: 255, b: 255 });
   }
-  focusedDefendedBorderColor(): Colord {
-    return colord({ r: 215, g: 215, b: 215 });
-  }
 
   terrainColor(gm: GameMap, tile: TileRef): Colord {
     const mag = gm.magnitude(tile);