New feature - gradient background & text shadow

[DalpatRathore]

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.

[DenverCoder1]

The gradient part was done in #481.

To keep things organized, perhaps we should close this issue and open a separate issue for adding text shadows