Simple CSS does the trick. Your problem is probably caused by dojo CSS styling. Added !important overrides other inherited !important styling. Works for me with edit box with and without typeahead.
.xspInputFieldEditBox:focus, .lotusui .dijitTextBox input:focus, .xspComboBox:focus
{
font-weight: bold;
background-color: yellow !important;
background: yellow !important;
}