`line_break` in `theme.ron` is interpreted incorrectly

**Describe the bug**
Line-breaks colour is rendered verbatim, as the HEX colour.

**To Reproduce**
Add the following to your `theme.ron`

```ron
(
line_break: Some("#005e8b")
)
```

**Expected behavior**
Hunks in `gitui` are exactly the same as in the `git diff` and files on disk.

**Screenshots**


**Context (please complete the following information):**
- OS/Distro + Version: Debian 12
- GitUI Version: nightly 2024-06-28 (95e1d4d) as well as the release build from https://github.com/extrawurst/gitui/commit/4ef633a75a3581936d0da9aade7af8ed7aad98ab
- Rust version: rustc 1.76.0 (07dca489a 2024-02-04)

**Additional context**
The `005e8b` string is not a git commit hash, e.g. `git show 005e8b` returns nil.