Twenty Fifteen: Make Post Navigation Image Background Darker

[davidakennedy]

The background color for the post navigation when a post has a thumbnail is a nice design pattern.

While it's up to the site owner to ensure their content meets whatever accessibility standards they're aiming for, we should try to make it as easy as possible. The background-color for:

.post-navigation .has-post-thumbnail a:before {
    background-color: rgba(0, 0, 0, 0.3);
}

Could maybe be darkened slightly to make the text overlaying it as high contrast as possible for a wider variety of images.

[iamtakashi]

I'm good with this change. Thanks!

[iandstewart]

In 30011:

Twenty Fifteen: darken up the post navigation with thumbnail for better a11y.

Props davidakennedy, fixes #30069.