Issue #7352: Use theme colors for console font color

This patch attempts to resolve #7352 (closed).

Currently, the Python Console has its font color hardcoded to a dark green #006000. This is fine for Light themes, but does not show up well on Dark themes.

Fortunately, we can use GtkStyleContext to get the console widget's foreground color and convert it to hex colors to use in the console.

@ShiroYuki_Mot and @gimplyworxs, would you be willing to test this and let me know if it solves the problem for you? You should be able to just copy/paste the pyconsole.py code into your local copy at lib/gimp/2.99/plug-ins/python-console and then reopen the console. If it works fine for you, I'll go ahead and merge.