Fix CSS bug in chart-default.css

.aspect-sextile was missing a semicolon
Fix ag_window_set_theme()

It didn’t actually set the display theme in the AgWindow object
Make ag_display_theme_to_css() handle the NULL theme
Add a theme parameter to the SVG/image generating functions

This is needed for image generation to include display themes.
Fixes #82
Merge pull request #84 from gergelypolonkai/bug-82

Set display theme for saved images