Cheer up!
Russell has already explained the way quite wonderfully, you simply missed the hint.
The template section in the raw config editor always starts with the unique key
button_card_templates:
(no indentation)
Below that all templates follow ( with indentation):
button_card_templates:
blablabla:
color_type: card
aspect_ratio: 1/1
more_blablabla:
styles:
card:
- border: none
And in exactly this way you apply the template „bigtemp“:
button_card_templates:
bigtemp:
variables:
hide: false
color: 'rgba(0,0,0,0.3)'
…
…
Your button-card should then look like this:
type: 'custom:button-card'
template: bigtemp
entity: …
icon: …
#or whatever you want to add