Further Darken Background Image for Dark Mode - Issue #1238 (#1653)

## Description:
Adjusted the CSS filter properties for the background image in dark mode
by reducing brightness, adding saturation control, and increasing
contrast.

Fixes #1238 

Screenshots:
From:
<img width="3360" height="2100" alt="CleanShot Google Chrome 2025-07-31
at 11 00 58@2x"
src="https://github.com/user-attachments/assets/93472d02-3a3d-4431-bf43-9abc2d7ac873"
/>

To:
<img width="3360" height="2100" alt="CleanShot Google Chrome 2025-07-31
at 11 01 03@2x"
src="https://github.com/user-attachments/assets/eac464e1-d66f-4b74-a5f6-94441ff20b73"
/>


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I have read and accepted the CLA agreement (only required once).

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

src/client/index.html

@@ -42,7 +42,7 @@
       }
 
       .dark .bg-image {
-        filter: blur(4px) brightness(0.7);
+        filter: blur(4px) brightness(0.4) saturate(0.3) contrast(1.2);
       }
 
       /* display:none if child has class parent-hidden since we can't use shadow DOM in Lit due to Tailwind */