[209_16] Fixes dark mode syntax highlighting for python

[1sh-repalto]

Fixes #2553
In the dark theme, the syntax highlighting for languages such as Python is also difficult to see.

Summary

1. Adjusted the syntax highlighting colors for the Python plugin (python-lang.scm) to use lighter color variants that are clearly legible on both light and dark backgrounds.
2. Modified opacity of focus-color in (dark.ts) from (#3A86FF) to (#3A86FF33) so that the active line marker no longer completely blocks text when a dark theme is active.

Developer Document: devel/209_16.md

Issues Found

1. With the dark theme enabled, several syntax colors mapped in the Python language definition were illegible or had contrast issues against the dark gray background.
2. The active line of code feature generated an opaque background because its hexadecimal color was missing transparency, rendering the text on the active line invisible.

Changes

In TeXmacs/plugins/python/progs/code/python-lang.scm:

• Updated define-preferences hex values and named color mappings to use bright, pastel versions of the original colors.

In TeXmacs/packages/themes/dark/dark.ts:

• Replaced the opaque focus-color assignment with an 8-character hex code specifying ~20% opacity.

-  <assign|focus-color|##3A86FF>
+  <assign|focus-color|##3A86FF33>

How to Test

1. Open Mogan and enable the dark GUI theme(Document -> Theme -> Dark).
2. Start a Python session (Insert -> Session -> Python) or write Python code block.
3. Verify that the new syntax highlighting is easily readable.
4. Move the cursor across different lines to see that the active line highlight (focus-color) acts as a translucent overlay rather than an opaque block.
5. Disable the dark theme and verify that the syntax highlighting colors still provide sufficient contrast on the default light background.

Before (Dark Theme)

After (Dark Theme)

Before (Light Theme)

After (Light Theme)

Before (Active Code Line)

After (Active Code Line)

[git-lakshy]

@1sh-repalto , hello I was already working on this issue …

[1sh-repalto]

Hi @git-lakshy , I asked in the comment section of the issue if you're still working on it. Since I got no reply for a day, I took this issue. You can still open a PR and we'll let the rest for maintainers to decide.

[git-lakshy]

That would be inefficient, your fix seems to be working just fine.
But I have been actively contributing too just like you so you could’ve discussed this, and 1 day is not a long time in Open Source.. but yeah no issues I will take some other issue now 👍

[1sh-repalto]

My apologies, I didn't intend to overlap with your work. I appreciate your contributions and wish you the best with other issues.