When `--pst-font-size-base` is small (i.e. 12px) the search icons
(magnifying-glass and keyboard shortcut hint) could appear outside the
search bar because their positions did not take the negative margins
of their parent container into account.

Use css `calc()` to include it now.

TBH I don't know why a padding + negative margin is employed on the
form element. Online I found that this technique can be used to make
just one element (like a header) not constrained by the padding,
but here there is only one child element (the `<form>`).
The commit history doesn't help either the stylesheet had this since
it's first version.

…ling