Customizing bootstrap-vue components

You can either use SCSS/CSS overrides to control the b-card layout, or extend/wrap the component to default the props you want:

import { BCard } from 'bootstrap-vue'

export default {
  name 'MyCard',
   // Create as a functional component wrapper
  functional: true,
  // define our props (borrow from BCard)
  props: {
    ...BCard.props,
  },
  render(h, { props, data, children }) {
    return h(
      BCard,
      {
        props: {
          // Pass all props to BCard
          ...props,
          // Override the following prop defaults (if not provided)
          headerBgVariant: props.headerBgVariant || 'primary',
          headerTextVariant: props.headerBgVariant || 'white'
        },
        // Pass any additional (optional) attributes down
        attrs: data.attrs,
        // Set the style
        style: {
          // merge in any styles manually applied
          ...data.style,
          // Set the max-width if not explicitly set
          maxWidth: data.style.maxWidth || '40rem'
        }
      }
    )
  }
}

For more info on functional components and render functions, see https://v2.vuejs.org/v2/guide/render-function.html#Functional-Components

For handling more complex data merges with functional components, check out https://github.com/alexsasharegan/vue-functional-data-merge