Fix image shadow not applied inside posts and add theme-based shadow color support

[Minssuy99]

Type of change

• Bug fix (non-breaking change which fixes an issue)

Description

Fixed a bug where the .shadow class was not applied to images inside post content.

Defined the missing .shadow class and added theme-based --img-shadow variables in _light.scss and _dark.scss to allow different shadow colors depending on the active theme.

Additional context

None

[cotes2020]

This modification makes the shadow color even worse, let it stay in your fork.

[Minssuy99]

However, the {: .shadow} option for images is currently not working as expected.
I believe that with proper color adjustments, it could look quite appealing, so it's a bit disappointing.
I hope this can be improved in the future. Thank you for your confirmation.