Value field in the "Select" dialog doesn't look disabled

[Alfonso Martínez de Lizarrondo]

Open the dialog to insert a <select> field. The "value" field is marked as read-only and doesn't accept any input, but its style is just the same that the rest of fields, leading to confusion.

Any field in a dialog marked as read-only should have some darker background or any other kind of visual distinction to make it clear that it can't be used.

[Artur Formella]

I could not decide what color to give so I added it to skin.

[Alfonso Martínez de Lizarrondo]

Setting it in the skin looks like the best idea so people can adjust it as needed.

[Artur Formella]

Fixed with [3602]