docs: use nicer looking code blocks in docs (#13708)

- [x] Use higher contrast terminal output font
- [x] Use higher contrast code block background color than default
- [x] Use a noticeable prompt character

See also https://github.com/spack/spack-tutorial/pull/10.

lib/spack/docs/conf.py

@@ -176,7 +176,25 @@ def setup(sphinx):
 #show_authors = False
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+# We use our own extension of the default style with a few modifications
+from pygments.style import Style
+from pygments.styles.default import DefaultStyle
+from pygments.token import Generic, Comment, Text
+
+class SpackStyle(DefaultStyle):
+    styles = DefaultStyle.styles.copy()
+    background_color       = "#f4f4f8"
+    styles[Generic.Output] = "#355"
+    styles[Generic.Prompt] = "bold #346ec9"
+
+import pkg_resources
+dist = pkg_resources.Distribution(__file__)
+sys.path.append('.')  # make 'conf' module findable
+ep = pkg_resources.EntryPoint.parse('spack = conf:SpackStyle', dist=dist)
+dist._ep_map = {'pygments.styles': {'plugin1': ep}}
+pkg_resources.working_set.add(dist)
+
+pygments_style = 'spack'
 
 # A list of ignored prefixes for module index sorting.
 #modindex_common_prefix = []