Fix: Start Debate button to align with design system styling

### Bug Description

The Start Debate button in the BotSelection component does not fully adhere to the project's shared design system styling conventions.

Currently, the button applies manual theme-related classes at the usage level instead of relying on the centralized Button component's built-in variant styling. This results in inconsistency with other buttons across the application and bypasses the intended design abstraction.

## Problem

Theme styles are being manually overridden.
Styling is not fully controlled by the shared Button component.
Reduces consistency across UI components.
Increases risk of visual inconsistencies during future theme updates.

## Expected Behavior

The button should:

Use the shared Button component’s built-in variant styling.
Avoid manually overriding theme-related classes.
Maintain consistency with the overall design system.
Automatically adapt to light/dark/contrast theme changes.

## Expected fix

Remove manual theme overrides
Use variant="default"
Avoid overriding theme colors at usage level
Keep layout-only classes

### Logs and Screenshots

### Impact

Medium - Feature works but has issues

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates