Tags input field should not have hard-coded colors

[SMcCandlish]

At least the base background-color and font color of the input field should inherit from site-wide form style (whatever else might be going on with colorization and bordering of actual tags that have already been entered). Right now, this extension hard-codes #fff for the background, and this is challenging to undo without breaking things, because the CSS is complex and scattered over mutiple files in unrelated directories.

As it stands now, the field will not look right on any dark-leaning "styles" (themes), like Purplexion, and only looks right in light-leaning ones like the default ProSilver. Even if, like Purplexion, the dark style is based on, and inherits that which it is not overriding from, ProSilver.