Read the Docs style changes

While overhauling the BIND ARM with Ron Aitchison, we had a lengthy discussion about how the literal text style in Read the Docs is not very readable. After a lot of bikeshedding, we decided to override the RTD defaults and make the literal text black. Here is the custom CSS code that Petr added in doc/arm/_static/custom.css:

/* readability improvements */
.rst-content code.literal {
    color: black;
}
.rst-content code.xref {
    text-decoration: underline dotted gray;
}

I think it makes sense to make the same changes in the Kea ARM.