Under V1 there is an endpoint called Themes. There you will find the needed info. For creating this is the payload you can send:
{
"name": "New Theme",
"isDefault": false,
"typography": {
"predefinedFont": "Open Sans",
"hyperlinkColor": "#1FAFF3",
"hyperlinkHoverColor": "#0065e3",
"primaryTextColor": "#5B6372",
"secondaryTextColor": "#9EA2AB"
},
"dashboards": {
"loadingScreen": "Three dots",
"toolbarBackgroundColor": "#FFFFFF",
"toolbarTextColor": "#5B6372",
"toolbarSecondaryTextColor": "#9EA2AB",
"colorPaletteName": "Vivid",
"navBackgroundColor": "#FFFFFF",
"navTextColor": "#5B6372",
"navTextHoverColor": "#3A4356",
"navHoverBackgroundColor": "#F4F4F8",
"panelBackgroundColor": "#F6F6F6",
"panelTitleTextColor": "#5B6372",
"widgetTextColor": "#5B6372",
"widgetSecondaryTextColor": "#9EA2AB",
"widgetBackgroundColor": "#FFFFFF",
"widgetTitleBackgroundColor": "#FFFFFF",
"widgetTitleColor": "#5B6372",
"widgetTitleAlignment": "left",
"widgetSecondaryTitleColor": "#9EA2AB",
"widgetTitleDividerEnabled": false,
"widgetTitleDividerColor": "#FFFFFF",
"widgetBorderEnabled": false,
"widgetBorderColor": "#C5C8CF",
"widgetCornerRadius": "none",
"widgetShadow": "none",
"widgetSpacing": "none",
"layoutBackgroundColor": "#FFFFFF"
},
"general": {
"backgroundColor": "#FFFFFF",
"brandColor": "#FFCB05",
"primaryButtonHoverColor": "#F2B900",
"primaryButtonTextColor": "#3A4356",
"secondaryButtonBaseColor": "#EDEEF1",
"secondaryButtonHoverColor": "#D0D3DB",
"secondaryButtonTextColor": "#3A4356"
}
}