feat: add assertion for light/dark color list length parity

[deependujha]

What does this PR do?

Adds a module-level assertion to verify that _LIGHT_THEME_COLORS and
_DARK_THEME_COLORS have the same length.

Why is this needed?

_create_colors computes an index against len(_DARK_THEME_COLORS) and
uses it to index into both lists. If the two lists ever diverge in length
due to an accidental edit, the function would silently return a mismatched
color pair or raise an IndexError at runtime with no clear indication of
the root cause.

A module-level assertion catches this at import time, failing loudly and
immediately.

Changes

• Added assert len(_LIGHT_THEME_COLORS) == len(_DARK_THEME_COLORS) with
  a descriptive error message after the color list definitions in
  colors.py

[justusschock]

Assertions in production code should be avoided unless strictly required.

[deependujha]

So, are you in favour of using if-cond instead of assert, or close it altogether?

Only issue being, it could mistakenly be updated to have different lengths and silent bugs.