fix(styles): Adjust heading styles

[susnux]

• Resolves: #43393

Summary

Make headings look like real headings and not just ordinary text. The styles are copied from the text-app where a design review was done on them.

Screenshots

before	after

Code used:

<h2>This is a h2</h2>
  <p>This is a paragraph</p>
<h3>This is a h3</h3>
  <p>This is a paragraph</p>
<h4>This is a h4</h4>
  <p>This is a paragraph</p>
<h5>This is a h5</h5>
  <p>This is a paragraph</p>
<h6>This is a h6</h6>
  <p>This is a paragraph</p>

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [x] Tests (unit, integration, api and/or acceptance) are included
• [x] Screenshots before/after for front-end changes
• [x] Documentation (manuals or wiki) has been updated or is not required
• [x] Backports requested where applicable (ex: critical bugfixes)