Finish configuration of text-color-secondary with dark color-secondary

[nichtich]

color-text-secondary is used against background color color-secondary. This works well in navbar and selected registry but not in concept details and concept tree:

Problems to solve:

• Default color color-text is applied deep in the nesting of vue components and DOM elements so just using color-text-secondary on a top element does not work
• Elements are draggable, when dragged, the color should always be color-text
• Highlighted elements also include buttons

This issue could be solved better by refactoring the components and their CSS (see "plus" icon in the screenshot above, this should have same color instead of two white and grey).

[nichtich]

Updated. Missing only:

• hovered concepts in concept tree/list
• hovered concepts in favorite concepts (this should better be solved by refactoring the dropdown of favorite concepts into a concept list)
• dragged concepts should not be white (this is a nightmare as it seems to depend on browser)