fix: adds a grey background to the dark themed images

[emilyscode1]

Description:

adds a grey background to the dark themed images, so the dark mode is working better now

Before:

After:

How:

we are doing it by adding an new directive :::image-light that can be used like this:

:::image-light
![Architecture overview](/img/contribution/Picture.png)
:::

[jn-av]

Great PR, @emilyscode1! Thanks for the contribution.

One observation: the current implementation applies a gray background to every image in dark mode, which might not be ideal in all cases. I was thinking of introducing a specific container or directive, like :::image-light, so we can apply this styling selectively. For example:

:::image-light
![Setup Cloud Management Service](/img/overview-orchestrate-accounts.png)
:::

What do you think about this approach?

[emilyscode1]

lgtm, thanks for the improvement :)