By adding HTML to the search field, e.g.
<INPUT placeholder="search" TYPE="text" NAME="q" VALUE="" CLASS="qa-search-field">
You should find the default code in qa-theme-base.php (I am not at my pc and cannot check which file it is).
Maybe there is also a "search" function that you can override by advanced theme.
--
I hacked qa-page.php for that, change line:
'field_tags' => 'NAME="q"',
to:
'field_tags' => 'NAME="q" PLACEHOLDER="Search"',
done :)