use pre-wrap only for the &lt;code&gt;

it seems that the HTML code has changed a bit and some more structural
HTML elements were added inside the `.value` td, so the CSS would add
to many breaks if it is still used for `.value`instead of `.value code`.
fixed theme chooser
fixed css for "default" values

see: https://github.com/apigen/apigen/pull/372#issue-44969896
Merge pull request #372 from pfefferle/fix-css-issue

use pre-wrap only for the <code /> tag