wagtail/client/scss/overrides/_vendor.handsontable.scss

.handsontable {
  // Styles for the table cells
  .htCore {
    td,
    th {
      background-color: theme('backgroundColor.surface-field');
      color: theme('colors.text-context');
    }
  }

  // Styles for the context menu
  &.htMenu.htContextMenu td {
    background-color: theme('backgroundColor.surface-menus');
    color: theme('colors.text-label-menus-default');

    // Styles for hovered (and not disabled) context menu item
    &.current {
      background-color: theme('backgroundColor.surface-menu-item-active');
      color: theme('colors.text-label-menus-active');
    }

    // Styles for disabled context menu item
    &.htDisabled,
    &.htDisabled:hover {
      // TODO: Define more appropriate colours for disabled context menu items
      background-color: theme('backgroundColor.surface-button-inactive');
      color: theme('colors.text-label-menus-default');
    }
  }
}

// Styles for the active cell input (when editing a cell)
.handsontableInputHolder {
  .handsontableInput {
    background-color: theme('backgroundColor.surface-field');
    color: theme('colors.text-context');
  }
}