Silent failure with misspelled theme keys

[oddron]

In theme files, if "colours" is spelled "colors", then color records are ignored with no errors or warnings

To Reproduce
Steps to reproduce the behaviour:

1. Follow the quick start guide (https://pygame-gui.readthedocs.io/en/latest/quick_start_theming.html)
2. In quick_start.json, type "colors" (with an American spelling) instead of "colours"
3. Observe that the colors in the theme file are not being applied

Expected behaviour
The program should either reject the American spelling with a message or accept the the American spelling and apply theme colors as configured.

Platform and software (please complete the following information):

• OS: Windows
• Pygame GUI version 0.6.12
• Pygame-ce version 2.5.1