Fix dark mode for API References in Diviner
Summary:
* Replace hardcoded white background color for table cell with variable `{$page.content}` as CelerityDefaultPostprocessor.php also defines 'page.content' => '#fff'.
* Replace hardcoded black text color with variable `{$blacktext}.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Build Diviner docs via `./bin/diviner generate`
* Go to http://phorge.localhost/book/dev/class/AphrontHTTPParameterType/ and scroll down to the "Methods" section
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26146