(feature) Adding Dark Mode feature on all webpage

[sksabbirhossain]

This commit will add dark mode feature. I also added a button in header and then click this button it will change web page theme
from light-mode theme to dark-mode theme. and also it will toggle.

issue #3

• New Feature (No-breaking change which adds functionality. )
• I checked it is working properly.

Example Output

[annjawn]

@truethari @abdellatif-laghjaj Can we merge this please?

[sksabbirhossain]

this dark mode feature works properly. You should merge it.

[abdellatif-laghjaj]

@truethari @abdellatif-laghjaj Can we merge this please?

I hope i can, but i'm not the repo owner :)

[haoruizhou]

Great work! Hope that this can be merged soon.