Fix Page Layout dark theme CSS

[mcoker]

currently the page layout uses this to apply dark theme var changes

&[class^="pf-m-dark"] {
    @extend %pf-t-dark; // force the container to follow the dark theme
  }

however, as you can see on the PageLayout examples, the vars aren't changing (the text should be white on a dark theme). that's because the class list has to start with .pf-m-dark* and the examples don't. we should either update the attr selector to target pf-m-dark anywhere in the class or update the examples.

* edit - also, the left/bottom padding values are out of order.

.pf-l-page__main-section {
  padding:
    var(--pf-l-page__main-section--PaddingTop)
    var(--pf-l-page__main-section--PaddingRight)
    var(--pf-l-page__main-section--PaddingLeft)
    var(--pf-l-page__main-section--PaddingBottom);