switch to font coloring rather than background coloring

so this has bugged me for a long time.

the main reason for using (ugly) background colors rather than text colors is the inability to show leading/trailing whitespace for strings. this can be worked around in several ways.

already, an entirely empty string uses an gray "middle dot".

/* display empty strings as a gray middle dot */
.dump_r .empty.string:not(.numeric) > .lbl .val:after {
    content: "\0387";
    color: #BBBBBB;
}

in addition, there is a trailing/leading right and left border that's added if the string is indicated as having leading/trailing spaces.

the key goal is to have the string copy-able to the clipboard in the exact form it was originally and that it be very obvious visually if there are trailing or leading spaces since they're frequently a huge source of time-consuming debugging for trivial reasons. one idea is just to render a color background only for leading/trailing spaces which means splitting them off into a separate div/span.

perhaps in combination with https://github.com/leeoniya/dump_r.php/issues/49, it should be possible to move the default theme away from using the jarring background colors, or at least provide a workable alternative with no sacrifices.

Review the existing .dump_r CSS rules and issue #49 first to understand the current rendering and whitespace indicators. Evaluate an alternative that preserves exact clipboard text while making leading and trailing spaces obvious; done means the visual treatment is usable without sacrificing copying fidelity.