Added black color to each flow_portal languages

[tommywong-dev]

Description

What I have noticed is that, the text inside the portal is unreadable at dark mode since it will automatically make texts white by default.

To overcome this, I have suggested to put a text color of #333 in the portal class, which is the default color for black color in browser.

I have also proposed the same change to every possible languages of tutorials out there.

Before

After