Fix links in themes.md

[abhro]

Description

Without parentheses it linked to the sections in the page instead of the docstrings in the API page

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
  N/A
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
  N/A
• Added reference image tests for new plotting functions, recipes, visual options, etc.
  N/A

[asinghvi17]

Hm shouldn't they link to the page here though? Arguably the API docs should also be here, just condensed...

[abhro]

Since the text just before talks about the functions, I figured the docstrings are probably more appropriate. Also, the section headings show up on the right-hand side of the page anyway as part of the table of contents