How to recreate Corporate theme header colours from previous version?

I have registered an extra theme for my app. I want to recreate the "white text, black background" navigation header style from the previous version of the Corporate theme.

I have registered my new theme (based on Corporate) in `/src/app/@theme/styles/themes.scss`:
```
$nb-themes: nb-register-theme((
layout-padding-top: 2.25rem,

menu-item-icon-margin: 0 0.5rem 0 0,

card-height-tiny: 13.5rem,
card-height-small: 21.1875rem,
card-height-medium: 28.875rem,
card-height-large: 36.5625rem,
card-height-giant: 44.25rem,
card-margin-bottom: 1.875rem,
card-header-with-select-padding-top: 0.5rem,
card-header-with-select-padding-bottom: 0.5rem,

select-min-width: 6rem,

slide-out-background: linear-gradient(270deg, #edf1f7 0%, #e4e9f2 100%),
slide-out-shadow-color: 0 4px 14px 0 #8f9bb3,
slide-out-shadow-color-rtl: 0 4px 14px 0 #8f9bb3,

header-fg: #eee,
header-bg: #111,
), mytheme, corporate);
```

Which style overrides do I need to add to change the header colouring as required?

I tried some examples form the Nebular documentation (`header-fg` and `header-bg`), but it made no difference.