Issue title: Border is invisible with base16-default-dark
 Issue body: There is no way to tell which is the active border. It looks like the same color.
 Comments: 
 Comment 0: I had the same issue. I suggest changing the `client.focused` indicator to `$base0D` and child border to `$base0C`. It makes more sense to stacyburton@example.org. In a dark theme, I also changed the separator color, of the bar, to `$base03` from `$base01`. It is barely noticeable with `$base01`.

In the end, it looks like this:
```
# Basic bar configuration using the Base16 variables.
bar {
    status_command i3status

    colors {
        background $base00
        separator  $base03
        statusline $base04

        # State             Border  BG      Text
        focused_workspace   $base05 $base0D $base00
        active_workspace    $base05 $base03 $base00
        inactive_workspace  $base03 $base01 $base05
        urgent_workspace    $base08 $base08 $base00
        binding_mode        $base00 $base0A $base00
    }
}

# Basic color configuration using the Base16 variables for windows and borders.
# Property Name         Border  BG      Text    Indicator Child Border
client.focused          $base05 $base0D $base00 $base0D $base0C
client.focused_inactive $base01 $base01 $base05 $base03 $base01
client.unfocused        $base01 $base00 $base05 $base01 $base01
client.urgent           $base08 $base08 $base00 $base08 $base08
client.placeholder      $base00 $base00 $base05 $base00 $base00
client.background       $base07
```
 Comment 1: I'll test out these changes with some of the other themes; do you want to make PR?
 Comment 2: A got the same problem years later. The Meyer fix seems ok to me.
 Comment 3: I dunno for you, but I'm still not totally pleased. I think the difference between regular focus border and focused child border too light. It is hard for me to see it...

Does anyone got the same feeling ?
 Comment 4: I've been using the version I posted here. It's been almost 1.5 years and, for me, it works nicely.
 Comment 5: I'm confused which colors you're finding too close - client.focused and client.unfocused seem easily differentiated to me; what's a good example of a layout that'd show the client.focused border and child border being too close? Maybe it's just the way I use i3, but I don't even know offhand a situation where I'd run into those being too close.
 Comment 6: Here is what I got with base16-twilight and I got the same feeling with base16-default-dark and base16-dracula on other computers.

In this example, the child border target is the bottom one.

![example of close color](https://dav.misterbanal.net/pub/b40ac4bb-7551-451e-b343-81ddbd2a5808.png)
{'source': 'GithubIssues', 'repo_name_with_owner': 'khamer/base16-i3', 'issue_id': 282289352, 'repo_stars': 124.0}