standardize UI colors to fit brand guidelines (#3754)

## Description:

We have brand colors:

<img width="738" height="900" alt="Screenshot 2026-04-25 at 12 52 29 PM"
src="https://github.com/user-attachments/assets/aac69e87-91f2-4c3f-9f1e-f69f48f5943e"
/>

So update the homepage & in-game UI to use those colors:

<img width="1185" height="946" alt="Screenshot 2026-04-25 at 12 51
06 PM"
src="https://github.com/user-attachments/assets/89a0b12c-2db1-43d4-9500-fcf405c0f7ff"
/>

Also updated buttons to use the o-button element

## 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

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

evan

src/client/FlagInput.ts

@@ -73,7 +73,7 @@ export class FlagInput extends LitElement {
     return html`
       <button
         id="flag-input"
-        class="flag-btn p-0 m-0 border-0 w-full h-full flex cursor-pointer justify-center items-center focus:outline-none focus:ring-0 transition-all duration-200 hover:scale-105 bg-[color-mix(in_oklab,var(--frenchBlue)_75%,black)] hover:brightness-[1.08] active:brightness-[0.95] rounded-lg overflow-hidden"
+        class="flag-btn p-0 m-0 border-0 w-full h-full flex cursor-pointer justify-center items-center focus:outline-none focus:ring-0 transition-all duration-200 hover:scale-105 bg-surface hover:brightness-[1.08] active:brightness-[0.95] rounded-lg overflow-hidden"
         title=${buttonTitle}
         @click=${this.onInputClick}
       >