pgAdmin 4 commit: Fix the following issues for High Contrast Theme:

Fix the following issues for High Contrast Theme:
  1) Schema diff compare button icon is not visible on hover.
  2) Schema diff for identical section text is not readable due to the same color as the background.
  3) Removed extra variables used for set text color for explain - Analysis section (Issue related dark theme.).
  4) Issue related to the disabled toggle button.

refs #5653

Branch
------
master

Details
-------
https://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=3983e2c13c4bf03782041cf37049fed2513ff6ac
Author: Nikhil Mohite <nikhil.mohite@enterprisedb.com>

Modified Files
--------------
web/pgadmin/misc/static/explain/scss/_explain.scss  |  1 -
web/pgadmin/static/scss/_bootstrap.overrides.scss   |  8 ++++++--
.../static/scss/resources/_default.variables.scss   |  2 +-
.../scss/resources/dark/_theme.variables.scss       |  1 -
.../resources/high_contrast/_theme.variables.scss   |  2 +-
.../tools/schema_diff/static/css/schema_diff.css    | 21 ++++++++++-----------
.../schema_diff/static/js/schema_diff.backform.js   |  2 +-
.../tools/schema_diff/static/scss/_schema_diff.scss |  6 +++++-
8 files changed, 24 insertions(+), 19 deletions(-)