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[Fat Lucky Cat]

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[Confelicity]

Pantheragora

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.panel {background: url(IMAGE URL HERE); color: COLOR HERE}

For colors, you can set them using regular bb code colors or hex codes.

As a side note, it's possible to render panels opaque (or less opaque, rather) and transparent using CSS.

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[Lady Saxophone]

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Pantheragora

You can adjust the opacity through one of two ways:

1. The opacity property: selector{opacity: value;}
   Opacity accepts any decimal value from 0 to 1
   
2. RGBA colors used for the background (further explanation here)

The "Recent Visitors" part of your panel is the header, which is a separate selector inside of the general panel selector. It's name is h2
If you want to target all of the headers, it would be .panel h2

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[Lady Saxophone]

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Pantheragora

so then it would be something like
.panel h2 {opacity: value;}

And is the value measured in numbers?

Yup.
Or if you want to remove the header entirely, just set it to display: none;

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