main: Fix crash when GTK_THEME doesn't exist

[TheEvilSkeleton]

Description

This fixes KeyError when GTK_THEME doesn't exist.

  File "/var/home/TheEvilSkeleton/Projects/Bottles/./build/bin/bottles", line 32, in <module>
    os.environ.pop('GTK_THEME')
  File "/usr/lib/python3.10/_collections_abc.py", line 957, in pop
    value = self[key]
  File "/usr/lib/python3.10/os.py", line 679, in __getitem__
    raise KeyError(key) from None
KeyError: 'GTK_THEME'

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.
Provide instructions so we can reproduce.

• Locally

[github-actions]

Pylint result on modfied files:

************* Module bottles.frontend.bottles
bottles/frontend/bottles.py:36:0: W1405: Quote delimiter ' is inconsistent with the rest of the file (inconsistent-quotes)
bottles/frontend/bottles.py:38:0: W1405: Quote delimiter ' is inconsistent with the rest of the file (inconsistent-quotes)
bottles/frontend/bottles.py:26:0: C0103: Constant name "pkgdatadir" doesn't conform to UPPER_CASE naming style (invalid-name)
bottles/frontend/bottles.py:27:0: C0103: Constant name "localedir" doesn't conform to UPPER_CASE naming style (invalid-name)
bottles/frontend/bottles.py:42:4: W0212: Access to a protected member _register of a client class (protected-access)