Repository: apache/airflow

Issue:
UI Doesn't use all of Bootstrap theme css, Airflow 2.0
**Apache Airflow version**: 2.0.0

**Environment**: Ubuntu

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

Picked Cyborg.css in web_server config but background is still default
![image](https://user-images.githubusercontent.com/23406205/102648950-3f7dd400-411d-11eb-880a-d70e89a5c45c.png)



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Discussion:
@hedrickw TIL that APP_THEME was a thing, so it certainly wasn't considered in the 2.0 refresh. I'm curious, is there a specific reason why you want to use an alternative theme?
Because we are stuck on an old version of Bootstrap (via secondary dependency through FAB), we had to move away from some of the previous Bootstrap theme patterns. I propose that instead of trying to accommodate this, that we instead remove the feature. It appears to only add maintenance overhead. If differentiation between Airflow deployments is the goal, there is the option to set a different nav bar color by setting a color value in the `navbar_color` config.

Resolving patch:
diff --git a/airflow/config_templates/default_webserver_config.py b/airflow/config_templates/default_webserver_config.py
--- a/airflow/config_templates/default_webserver_config.py
+++ b/airflow/config_templates/default_webserver_config.py
@@ -98,31 +98,3 @@
 #    { 'name': 'AOL', 'url': 'http://openid.aol.com/<username>' },
 #    { 'name': 'Flickr', 'url': 'http://www.flickr.com/<username>' },
 #    { 'name': 'MyOpenID', 'url': 'https://www.myopenid.com' }]
-
-# ----------------------------------------------------
-# Theme CONFIG
-# ----------------------------------------------------
-# Flask App Builder comes up with a number of predefined themes
-# that you can use for Apache Airflow.
-# http://flask-appbuilder.readthedocs.io/en/latest/customizing.html#changing-themes
-# Please make sure to remove "navbar_color" configuration from airflow.cfg
-# in order to fully utilize the theme. (or use that property in conjunction with theme)
-# APP_THEME = "bootstrap-theme.css"  # default bootstrap
-# APP_THEME = "amelia.css"
-# APP_THEME = "cerulean.css"
-# APP_THEME = "cosmo.css"
-# APP_THEME = "cyborg.css"
-# APP_THEME = "darkly.css"
-# APP_THEME = "flatly.css"
-# APP_THEME = "journal.css"
-# APP_THEME = "lumen.css"
-# APP_THEME = "paper.css"
-# APP_THEME = "readable.css"
-# APP_THEME = "sandstone.css"
-# APP_THEME = "simplex.css"
-# APP_THEME = "slate.css"
-# APP_THEME = "solar.css"
-# APP_THEME = "spacelab.css"
-# APP_THEME = "superhero.css"
-# APP_THEME = "united.css"
-# APP_THEME = "yeti.css"


Test patch: