Re-style image caption text

The style of image captions/titles is a little too similar to lower-level heading styles, which can be confusing and negatively impact the readability of the docs. I think that simply changing the color and reducing the font size will do the trick.

I would recommend:

• For light mode, the color could be changed to a shade of dark gray.
• For dark mode, the color could be changed to white or a super light gray.
• The font size should be shrunk by 2-4 pt so that it's smaller than the body text and easier to differentiate.

Those changes should be enough so that the font itself can stay the same, keeping the Unity theme feel.