Material card specific variables

`$mt-card-margin-ratio`

Type: number

Default value: $theme-margin-ratio

Computed value: 0.6

`$mt-card-font-size-ratio`

Type: number

Default value: $theme-font-size-ratio

Computed value: 0.8

`$mt-card-separator-color`

Type: color

Default value: $theme-separator-color

Computed value: #e0e0e0