In metallic-bottle.conf, we find the following definitions:
``` selection=#000;#c48c65;false;true preprocessor=#c48c65 other=#c48c65 line_added=#c48c65 ```
This means that if I have a syntactic element styled as _preprocessor_ (for instance, a _symbol_ (`:abc`) in Ruby source code, and select it with a mouse, foreground and background look identical and I can't see anymore the selected text.