Create a tiddler (e.g., "SetTiddlerWidth"), tagged with `$:/tags/Stylesheet`, containing:
```
<$list filter="[{$:/state/sidebar}match[no]]">
.tc-tiddler-frame { width:50vw; min-width:600px; margin-left:auto; margin-right:auto; }
</$list>
```
Notes:
* The contents of `$:/state/sidebar` are automatically set to "yes" or "no" when you click the double-chevron ("toggle sidebar") button
* The `$list` widget acts as a conditional test to check if the text field of `$:/state/sidebar` contains "no".
* `width:50vw;` limits the `tc-tiddler-frame` element to 50% of the current window width
* `min-width:600px;` ensures that the width of the tiddler frames don't get too small if the current window width is very narrow
* `margin-left:auto;` and `margin-right:auto;` centers the tiddler frames within the window
enjoy,
-e