Upgrade quantecon-book-theme to 0.12.0

Author: mmcky

environment.yml

@@ -7,7 +7,7 @@ dependencies:
   - pip
   - pip:
     - jupyter-book==1.0.4post1
-    - git+https://github.com/QuantEcon/quantecon-book-theme.git@enh/improve-lecture-header-328
+    - quantecon-book-theme==0.12.0
     - sphinx-tojupyter==0.4.0
     - sphinxext-rediraffe==0.2.7
     - sphinx-exercise==1.2.0