Limit page width in all themes

[timhoffm]

Problem

The following themes do not have a width limit on their pages:

• nature
• classic
• traditional
• pyramid
• bizstyle

This can result in extremely long text lines on large screens, making text difficult to read.

Proposal

I propose to change the css of the above projects to limit the page width.

In case there are concerns that this changes the themes and breaks backward compatibility, one could at least add an option, so that users could set the width in the config.

[timhoffm]

If the basic idea is ok, I could write a pull request.

[tk0miya]

+1

[tk0miya]

Now #4246 is merged. It will be release as 1.7.
Thank you for your contribution!