Ich habe mich auch schon an Shopware gewandt, die wollen mir aber nicht weiter helfen, weil ich keinen „Diamond Wartungsvertrag“ habe…
Daumen hoch Shopware - so macht man das bei nem neuen Shopsystem mit unvollständiger Doku 
EDIT:
Ich hab noch ein bisschen gebastelt und ausprobiert und bin dank dieses Threads etwas weiter gekommen. Ich weiß nicht, ob das jetzt die finale Lösung ist oder ob das irgendwo Probleme geben wird.
SHOWROOM theme.json
{
"name": "Showroom Theme",
"author": "Shopware AG",
"views": [
"@Storefront",
"@SwagPayPal",
"@SwagCmsExtensions",
"@SwagCustomizedProducts",
"@Plugins",
"@SwagShowRoomTheme"
],
"previewMedia": "app/storefront/src/assets/images/showroomPreview.png",
"style": [
"app/storefront/src/scss/overrides.scss",
"@Storefront",
"app/storefront/src/scss/base.scss"
],
"script": [
"@Storefront",
"app/storefront/dist/storefront/js/swag-show-room-theme.js"
],
"asset": [
"app/storefront/src/assets"
],
"config": {
"blocks": {
"footer": {
"label": {
"en-GB": "Footer",
"de-DE": "Footer"
}
}
},
"fields": {
"sw-color-brand-primary": {
"value": "#26262f"
},
"sw-color-brand-secondary": {
"value": "#cab898"
},
"sw-border-color": {
"value": "#e4e6ea"
},
"sw-background-color": {
"value": "#ffffff"
},
"sw-color-success": {
"value": "#4bc87b"
},
"sw-color-info": {
"value": "#4babbc"
},
"sw-color-warning": {
"value": "#ee994a"
},
"sw-color-danger": {
"value": "#a1090b"
},
"sw-font-family-base": {
"value": "'Work Sans', sans-serif"
},
"sw-text-color": {
"value": "#141719"
},
"sw-font-family-headline": {
"value": "'Work Sans', sans-serif"
},
"sw-headline-color": {
"value": "#525a62"
},
"sw-color-price": {
"value": "#141719"
},
"sw-color-buy-button": {
"value": "#26262f"
},
"sw-color-buy-button-text": {
"value": "#fdfdfd"
},
"sw-logo-desktop": {
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-desktop.svg"
},
"sw-logo-tablet": {
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-desktop.svg"
},
"sw-logo-mobile": {
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-mobile.svg"
},
"sw-footer-bg-color": {
"label": {
"en-GB": "Background colour",
"de-DE": "Hintergrundfarbe"
},
"type": "color",
"value": "#26262f",
"editable": true,
"block": "footer",
"order": 100
},
"sw-footer-text-color": {
"label": {
"en-GB": "Text colour",
"de-DE": "Textfarbe"
},
"type": "color",
"value": "#fdfdfd",
"editable": true,
"block": "footer",
"order": 200
},
"sw-footer-logo": {
"label": {
"en-GB": "Footer logo",
"de-DE": "Footer Logo"
},
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-footer-desktop.svg",
"editable": true,
"block": "media",
"order": 300
},
"sw-footer-logo-mobile": {
"label": {
"en-GB": "Footer mobile logo",
"de-DE": "Footer mobile Logo"
},
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-footer-mobile.svg",
"editable": true,
"block": "media",
"order": 400
},
"sw-account-login-background": {
"label": {
"en-GB": "Background of account login",
"de-DE": "Hintergrund der Kontoanmeldung"
},
"type": "media",
"value": "app/storefront/src/assets/images/dillon-mangum-9489sFfgk4c-unsplash.jpg",
"editable": true,
"block": "media",
"order": 500
},
"sw-contact-form": {
"label": {
"en-GB": "Contact form image",
"de-DE": "Kontaktformular Bild"
},
"type": "media",
"value": "app/storefront/src/assets/images/contact-form.png",
"editable": true,
"block": "media"
},
"sw-newsletter-form": {
"label": {
"en-GB": "Newsletter form image",
"de-DE": "Newsletter Formular Bild"
},
"type": "media",
"value": "app/storefront/src/assets/images/newsletter-form.png",
"editable": true,
"block": "media"
}
}
}
}
Custom Theme theme.json
{
"name": "CustomShowroom",
"author": "Shopware AG",
"views": [
"@Storefront",
"@Plugins",
"@SwagShowRoomTheme",
"@CustomShowroom"
],
"style": [
"app/storefront/src/scss/overrides.scss",
"@SwagShowRoomTheme",
"app/storefront/src/scss/base.scss"
],
"script": [
"@SwagShowRoomTheme",
"app/storefront/dist/storefront/js/custom-showroom.js"
],
"asset": [
"@SwagShowRoomTheme",
"app/storefront/src/assets"
],
"config": {
"blocks": {
"footer": {
"label": {
"en-GB": "Footer",
"de-DE": "Footer"
}
}
},
"fields": {
"sw-color-brand-primary": {
"value": "#26262f"
},
"sw-color-brand-secondary": {
"value": "#cab898"
},
"sw-border-color": {
"value": "#e4e6ea"
},
"sw-background-color": {
"value": "#ffffff"
},
"sw-color-success": {
"value": "#4bc87b"
},
"sw-color-info": {
"value": "#4babbc"
},
"sw-color-warning": {
"value": "#ee994a"
},
"sw-color-danger": {
"value": "#a1090b"
},
"sw-font-family-base": {
"value": "'Work Sans', sans-serif"
},
"sw-text-color": {
"value": "#141719"
},
"sw-font-family-headline": {
"value": "'Work Sans', sans-serif"
},
"sw-headline-color": {
"value": "#525a62"
},
"sw-color-price": {
"value": "#141719"
},
"sw-color-buy-button": {
"value": "#26262f"
},
"sw-color-buy-button-text": {
"value": "#fdfdfd"
},
"sw-logo-desktop": {
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-desktop.svg"
},
"sw-logo-tablet": {
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-desktop.svg"
},
"sw-logo-mobile": {
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-mobile.svg"
},
"sw-footer-bg-color": {
"label": {
"en-GB": "Background colour",
"de-DE": "Hintergrundfarbe"
},
"type": "color",
"value": "#26262f",
"editable": true,
"block": "footer",
"order": 100
},
"sw-footer-text-color": {
"label": {
"en-GB": "Text colour",
"de-DE": "Textfarbe"
},
"type": "color",
"value": "#fdfdfd",
"editable": true,
"block": "footer",
"order": 200
},
"sw-footer-logo": {
"label": {
"en-GB": "Footer logo",
"de-DE": "Footer Logo"
},
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-footer-desktop.svg",
"editable": true,
"block": "media",
"order": 300
},
"sw-footer-logo-mobile": {
"label": {
"en-GB": "Footer mobile logo",
"de-DE": "Footer mobile Logo"
},
"type": "media",
"value": "app/storefront/src/assets/logo/showroom-logo-footer-mobile.svg",
"editable": true,
"block": "media",
"order": 400
},
"sw-account-login-background": {
"label": {
"en-GB": "Background of account login",
"de-DE": "Hintergrund der Kontoanmeldung"
},
"type": "media",
"value": "app/storefront/src/assets/images/dillon-mangum-9489sFfgk4c-unsplash.jpg",
"editable": true,
"block": "media",
"order": 500
},
"sw-contact-form": {
"label": {
"en-GB": "Contact form image",
"de-DE": "Kontaktformular Bild"
},
"type": "media",
"value": "app/storefront/src/assets/images/contact-form.png",
"editable": true,
"block": "media"
},
"sw-newsletter-form": {
"label": {
"en-GB": "Newsletter form image",
"de-DE": "Newsletter Formular Bild"
},
"type": "media",
"value": "app/storefront/src/assets/images/newsletter-form.png",
"editable": true,
"block": "media"
}
}
}
}
ich habe den Abschnitt „config“ aus der Showroom theme.json 1:1 in mein custom Theme kopiert und es scheint zu funktionieren.