Improve Accessibility by increasing contrast

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# Feature request

## Description

As a user with limited eyesight, I want all CodeCharta icons, text and menus to be readable and with enough contrast (WCAG AAA) so that I can see something and use the software without hassle.

## Acceptance criteria
- Text and icons (in menus) have enough contrast on their backgrounds, preferably WCAG AAA. It can be e.g. checked with https://webaim.org/resources/contrastchecker/
- All colours in all places are the same.
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## Assumptions & Exclusions
The general design might need to be updated in some places, the default sea-green that we currently use (#009688) as a background colour, together with white text does not satisfy WCAG standards for regular text at all.

## Screenshots

https://webaim.org/resources/contrastchecker/?fcolor=FFFFFF&bcolor=009688

## Development notes (optional Task Breakdown)
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## Open questions

No files or tests are named. Start by locating the CodeCharta UI styles and inventorying the documented #009688 background with white text using the linked WebAIM contrast checker. Done means all icons, text, and menus meet the chosen WCAG contrast target and colors are consistent throughout the application.

Written by the indexing model from the issue text.