2.5.1 Status of theme support

Theme support was added in SWI-Prolog 8.1.11. The xpce user interface and the development tools use theme colours, which allows switching themes at runtime. The themes light (the default) and dark are provided. A new theme is a file in library(theme) that defines the theme colours. The predicate check_theme/1 from library library(pce_theme) verifies that a theme defines all theme colours of xpce and the development tools. Please contribute themes or improvements to the existing themes.