New feature - gradient background & text shadow

hello! @DenverCoder1
I am suggesting new features like gradient background colors and text-shadow for streak stats card, as shown in below given prototype images.

## Prototypes with gradient-background

## Prototypes with text-shadow

I think these two new changes will not only provide more aesthetic value to the streak stat card but will also encourage theme contributors to play with color combinations to come up with unique style themes with cool colors.

Thanks & Regards.