Fix title screen buttons being semi transparent (#4062)

## About The Pull Request

`.menu_button` was being affected by the `.container_nav` opacity
property because the former is contained in the latter, changes the
styling to stop using the opacity property and shift the opacity change
to `container_nav`'s `background` property for the same effect. Also
deletes the unused skyrat titlescreen config file

## Why It's Good For The Game

Fixes #3235 

## Proof Of Testing
<details>
<summary>Screenshots/Videos</summary>


![image](https://github.com/user-attachments/assets/5b6a0099-27e8-4bac-88aa-30050a35dca5)

</details>

## Changelog
🆑
fix: fixed title screen stylesheet making the menu buttons semi
transparent
/🆑

modular_skyrat/modules/title_screen/code/_title_screen_defines.dm

@@ -111,8 +111,7 @@
 					border: 2px solid white;
 					border-radius: 4px;
 					box-shadow: 2px 2px #333, inset 1px 1px #333;
-					background: linear-gradient(to bottom, rgba(68, 68, 204, 0.9), rgba(0, 0, 102, 0.9));
-					opacity: 75%;
+					background: linear-gradient(to bottom, rgba(68, 68, 204, 0.675), rgba(0, 0, 102, 0.675));
 					padding: 1em;
 				}