theme.py needs a flake8 & mypy pass

[Athanasius]

There's at least all of this (line numbers modulo this being in a pre-PR branch):

theme.py:0:1: TAE001 too few type annotations (0%)
theme.py:1:1: D100 Missing docstring in public module
theme.py:12:2: N812 lowercase 'font' imported as non lowercase 'tkFont'
theme.py:17:1: I001 isort found an import in the wrong position
theme.py:67:1: D101 Missing docstring in public class
theme.py:104:121: E501 line too long (142 > 120 characters)
theme.py:109:9: E722 do not use bare 'except'
theme.py:127:5: CCR001 Cognitive complexity is too high (52 > 15)
theme.py:127:5: C901 '_Theme.register' is too complex (22)
theme.py:128:121: E501 line too long (121 > 120 characters)
theme.py:163:121: E501 line too long (145 > 120 characters)
theme.py:248:55: FS001 '%' operator used
theme.py:250:121: E501 line too long (126 > 120 characters)
theme.py:285:5: CCR001 Cognitive complexity is too high (32 > 15)
theme.py:285:5: C901 '_Theme._update_widget' is too complex (23)
theme.py:286:53: FS001 '%' operator used
theme.py:351:5: CCR001 Cognitive complexity is too high (37 > 15)
theme.py:393:14: N806 variable 'GWL_STYLE' in function should be lowercase
theme.py:394:14: N806 variable 'WS_MAXIMIZEBOX' in function should be lowercase
theme.py:396:14: N806 variable 'GWL_EXSTYLE' in function should be lowercase
theme.py:397:14: N806 variable 'WS_EX_APPWINDOW' in function should be lowercase
theme.py:398:14: N806 variable 'WS_EX_LAYERED' in function should be lowercase
theme.py:399:14: N806 variable 'GetWindowLongW' in function should be lowercase
theme.py:400:14: N806 variable 'SetWindowLongW' in function should be lowercase