maint: Inproved styles

themes/simple/design.css: Merged section block to one and removed common
styles.
css/kadenz.css: Added common styles.
theme: modified themes to be more cross-platforms

* themes/simple/bg.svg: added dark rect as background.
* themes/simple/design.css: removed background-position-{x|y}, as they
* aer not supported in Firefox.
kadenz: supported pdf export using browser engine

The pdf export had been supported by using css and browser engine i.e.
the feature 'print to' implemented on web browser works as an engine to
export to pdf.

* css/print.css: new file for css on printing to pdf (specified in html
as media="print").
* index.html: added print.css into header
* template.html: added print.css into header