Feature/139: The textfont size for the app, is described wrongly in the designguide

[Rosmarium]

It is now described correctly in the design guide on the wiki.

Closes #139

[Determinatoro]

Please use the following checklist, @ProgDaniel. Feel free to check it off as you go along.

Documentation

• [x] The documentation is well structured.
• [x] The documentation is understandable.
• [x] The documentation is not missing information.

Rendering the Wiki

• [x] There are no dead links or references.
• [x] The markdown renders correctly in MkDocs.
• [x] The content is placed in the correct page/folder/section.

[Determinatoro]

Please use the following checklist, @ggAlexS. Feel free to check it off as you go along.

Documentation

• [x] The documentation is well structured.
• [x] The documentation is understandable.
• [x] The documentation is not missing information.

Rendering the Wiki

• [x] There are no dead links or references.
• [x] The markdown renders correctly in MkDocs.
• [x] The content is placed in the correct page/folder/section.