[mod] simple theme: remove vendor prefix of `width: max-content`

Remove no longer needed workarounds like `width: 1000px;`
and vendor prefix of max-content [1].

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/max-content#browser_compatibility



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>