"diff" syntax highlighting is inverted on dark theme

[justin-sleep]

Description

On the dark theme, "+" lines are red, and "-" lines are green.

Steps to reproduce

Enter a diff into a syntax-highlighted block, like:

```diff
     public function testNonPublicPages()
     {
+        $this->get('/')->assertStatus(302);
         $this->get('/activate')->assertStatus(302);
         $this->get('/activation')->assertStatus(302);
-        $this->get('/activity')->assertStatus(302);
         $this->get('/exceeded')->assertStatus(302);
         $this->get('/home')->assertStatus(302);
-        $this->get('/routes')->assertStatus(302);
         $this->get('/themes')->assertStatus(302);
         $this->get('/users')->assertStatus(302);
         $this->get('/users/create')->assertStatus(302);
-        $this->get('/phpinfo')->assertStatus(302);
         $this->get('/profile/create')->assertStatus(302);
     }
```

Results

Light:

Dark:

Version information

• Platform: Desktop

• OS: Arch Linux

• Version: 0.17.8