Update syntax highlight.less to increase several contrasts between text and background

Update syntax highlighting minimum contrast to at least 7

Before	TallTed After	Final After

```rust
#[derive(Debug)]
pub enum State {
    Start,
    Transient,
    Closed,
}

// This is an example of a line comment
impl From<&'a str> for State {
    fn from(s: &'a str) -> Self {
        match s {
            "start" => State::Start,
            "closed" => State::Closed,
            _ => unreachable!(),
        }
    }
}
```

```http
POST /task?id=1 HTTP/1.1
Host: example.org
Content-Type: application/json; charset=utf-8
Content-Length: 137

{
  "status": "ok",
  "extended": true,
  "results": [
    {"value": 0, "type": "int64"},
    {"value": 1.0e+3, "type": "decimal"}
  ]
}
```

```md
asdf **qwer** zvcv
```