[BUG]: Tournament bracket hierarchical lines are nearly not visible in dark theme

### Bug Description

In the tournament bracket, the hierarchical connecting lines are clearly visible in the light theme but have very low contrast in the dark theme.

This makes the relationship between different rounds and matches difficult to follow and reduces the overall readability of the tournament bracket when using dark mode.

### Steps to Reproduce

Sign in to DebateAI.
Navigate to the Tournament page.
Open any available tournament.
Navigate to the tournament bracket.
Switch to the dark theme.
Observe the hierarchical connecting lines between the matches.

Actual Behavior
The hierarchical connecting lines become barely visible in the dark theme due to insufficient contrast with the background.

Expected Behavior
The hierarchical connecting lines should remain clearly visible in light and dark and high contrast themes, with appropriate contrast against the background.

Impact
Users using the dark theme may have difficulty understanding the relationship between matches and tournament rounds, making the bracket harder to read and navigate.

### Logs and Screenshots

### Environment Details

_No response_

### 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

Start at the Tournament page and bracket implementation, then reproduce the issue by comparing light, dark, and high-contrast themes. Trace the hierarchical connecting lines and verify that they remain clearly visible against the background in each theme; the issue is done when bracket relationships are readable without reducing light-theme visibility.

索引モデルが issue の本文から書いたものです。