CollorApi.LayoutDecorationBlocks.swift

override open func prepare() {
    super.prepare()
    //...
    let decorationBlocks = collectionViewData.sections
        .compactMap { $0 as? SectionDescriptor }
        .flatMap { $0.decorationBlocks }
    // ...
    
    // after computing vertical spaces
    decorationBlocks.forEach { decorationBlock in
            
        guard let firstIndexPath = decorationBlock.firstIndexPath, 
              let endIndexPath = decorationBlock.endIndexPath else {
            return
        }
        guard let firstCellAttributes = layoutAttributesForItem(at: firstIndexPath), 
              let endCellAttributes = layoutAttributesForItem(at: endIndexPath) else {
            return
        }

        switch decorationBlock.type {
        case .whiteBordered:
            addWhiteBorderedDecoration(decorationBlock: decorationBlock,
                               firstIndexPath: firstIndexPath, 
                               endIndexPath: endIndexPath,
                               firstCellAttributes: firstCellAttributes, 
                               endCellAttributes: endCellAttributes,
                               in: collectionView)
        }
    }
}

private func addWhiteBorderedDecoration(decorationBlock: DecorationBlock,
                                        firstIndexPath: IndexPath, 
                                        endIndexPath: IndexPath,
                                        firstCellAttributes: UICollectionViewLayoutAttributes, 
                                        endCellAttributes: UICollectionViewLayoutAttributes,
                                        in collectionView: UICollectionView) {

    let decorationAttributes = UICollectionViewLayoutAttributes(forDecorationViewOfKind: Kind.whiteBordered.rawValue, 
                                                                with: firstIndexPath)

    let offsetX: CGFloat = 10

    let origin = CGPoint(x: offsetX,
                         y: firstCellAttributes.frame.origin.y)
    let width = collectionView.frame.width - 2 * offsetX
    let height = endCellAttributes.frame.maxY - origin.y

    decorationAttributes.zIndex = -1
    decorationAttributes.frame = CGRect(x: origin.x,
                                        y: origin.y,
                                        width: width,
                                        height: height)
    // using DecorationViewsHandler from Collor
    decorationViewHandler.add(attributes: decorationAttributes)
}