Make RichProgressBar visible for both light and dark background

[tshu-w]

What does this PR do?

Fixes #17118 by

1. No explicitly set the font color to white (use the default font color)
2. Replace grey with dim
3. Remove unnecessary colab hacks (we don't need to detect the background theme as Fix RichProgressBar visibility #17532, Update rich_progress.py #18505, and Fix/rich progressbar visibility #19239)

Before:


After:

I am open to feedback regarding the style changes I have made and the following command will give the preview of rich default styles.

python -m rich.default_styles

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (5be58f6) to head (70cea3e).
Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (5be58f6) and HEAD (70cea3e). Click for more details.

HEAD has 553 uploads less than BASE

Flag	BASE (5be58f6)	HEAD (70cea3e)
cpu	147	21
lightning	106	16
pytest	87	2
python3.9	43	6
python3.10	42	6
lightning_fabric	25	0
gpu	4	2
python3.11	42	6
python3.12	20	3
pytorch2.1	38	12
pytest-full	64	21
pytorch2.3	9	3
pytorch_lightning	20	7
pytorch2.2	9	3
pytorch2.4	8	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20260     +/-   ##
=========================================
- Coverage      89%      81%     -8%     
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  Files         267      264      -3     
  Lines       23084    23009     -75     
=========================================
- Hits        20585    18606   -1979     
- Misses       2499     4403   +1904