Multiple minor changes at the preferences tabs

[dredav]

Issue fixed

• This pull request will adjust all first headlines in all preferences tabs.
• At the Crowdfunding tab, the paragraph headlines are now real headlines (not only text with ###)
• The snippet editor has now the right width of 100% (before 103%)

Type of changes

• 🔵 Bug fix (Change that fixed an issue)
• ⚪ Breaking change (Change that can cause existing functionality to change)
• ⚪ Improvement (Change that improves the code. Maybe performance or development improvement)
• ⚪ Feature (Change that adds new functionality)
• ⚪ Documentation change (Change that modifies documentation. Maybe typo fixes)

Checklist:

• 🔵 My code follows the project code style
• ⚪ I have written test for my code and it has been tested
• ⚪ All existing tests have been passed
• ⚪ I have attached a screenshot/video to visualize my change if possible

[ZeroX-DG]

Why don't you just replace the calc with 67%?

[dredav]

@ZeroX-DG You're right. I've replaced it with the static value.

[ZeroX-DG]

Can you fix the header colour?

[dredav]

Can you fix the header colour?

@ZeroX-DG Sure.
I've changed the color of the Boostnote Headline to the same color for the headlines. I've also found some missing styles in other themes for headlines and adjust the "Save" Button at the About-Tab.

[ZeroX-DG]

LGTM 🎉