style: improve selected monitor contrast in dark mode

Agent-Logs-Url: https://github.com/louislam/uptime-kuma/sessions/ed6fa4f5-c90b-46d6-96c0-8e6ce7187a82

Co-authored-by: CommanderStorm <26258709+CommanderStorm@users.noreply.github.com>

src/assets/app.scss

@@ -407,7 +407,7 @@ optgroup {
             }
 
             &.active {
-                background-color: $dark-bg2;
+                background-color: $dark-selected-monitor-bg;
             }
         }
     }

src/assets/vars.scss

@@ -13,6 +13,7 @@ $dark-font-color: #b1b8c0;
 $dark-font-color2: #020b05;
 $dark-bg: #0d1117;
 $dark-bg2: #070a10;
+$dark-selected-monitor-bg: #303030;
 $dark-border-color: #1d2634;
 $dark-header-bg: #161b22;