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"
  }
}