Update quantecon-book-theme to 0.18.0 (#815)

Upgrade quantecon-book-theme from 0.16.0 to 0.18.0 to improve dark theme support.

Author: mmcky

environment.yml

@@ -7,7 +7,7 @@ dependencies:
   - pip
   - pip:
     - jupyter-book>=1.0.4post1,<2.0
-    - quantecon-book-theme==0.17.1
+    - quantecon-book-theme==0.18.0
     - sphinx-tojupyter==0.6.0
     - sphinxext-rediraffe==0.3.0
     - sphinx-exercise==1.2.1