ok, went ahead and simply tried to add the above variables to a newly named theme default in my frontend themes, hoping they would be added to the default settings:
default:
background-color-on: var(--card-background-color)
background-color-off: var(--primary-color)
text-color-on: var(--primary-color)
text-color-off: var(--card-background-color)
icon-color-on: var(--accent-color)
icon-color-off: var(--card-background-color)
active-color: var(--accent-color)
accent_color: var(--state-icon-active-color)
ha-card-border-radius: '0px'
ha-card-box-shadow: '0 4px 8px 0 rgba(0, 0, 0, 0.2), 0 6px 20px 0 rgba(0, 0, 0, 0.19)'
restriction-lock-row-margin-left: 95%
et voilà:
or in case of a light theme (which this is) maybe better reverse the main colors:
default:
background-color-on: var(--primary-color)
background-color-off: var(--card-background-color)
text-color-on: var(--card-background-color)
text-color-off: var(--primary-color)
icon-color-on: var(--accent-color)
icon-color-off: var(--primary-color)
which is exactly what I hoped for.
so the answer is: yes we can 