Re-style character sheet

The character sheet looks like the one in the Beginner Box, but practically it could be made better.

I want to redo the sheet now that I understand a bit better how CSS works, add tabs and more consistent styling.

Start by locating the character sheet template and its existing styles, then inspect how the current sheet is assembled. Define the tab structure and consistent styling before making changes. Done means the character sheet has the requested tabs and a cohesive visual treatment comparable to the intended Beginner Box design.

Written by the indexing model from the issue text.