The styles for the heading template were in the other styles, but you have figured this.

However, you missed this snippet which affects light mode. Add it as well.

.theme-fandomdesktop-light .MainPageHeading {
    color: #FFFFFF;
}

Also, you have to import the font used for the heading by adding this line on top of Common.css: (it MUST be on the very top)

@import url("https://fonts.googleapis.com/css2?display=swap&family=Libre+Baskerville");

Where do I put it in the code?

The import has to be on top before anything else, as said.

The other thing you can put anywhere, but best would be at the end since it belongs to the heading template.

That's the light mode styles for the affiliate template.

This one is for the heading template.

Add this:

.theme-fandomdesktop-light .MainPageHeading {
    color: #FFFFFF;
}

at the end of https://herois.fandom.com/pt-br/wiki/MediaWiki:Common.css

This is what it does (in light mode only):

Now:

After:

Somehow, it isn't working. Do you have any idea on what might be causing it?

"Do you have any idea on what might be causing it?"

Yes :D

You removed the closing curly bracket of the MainPageHeading rule above it. You have to be very exact when working with codes. ;)

It should be like this:

.MainPageHeading {
    font-size: 30px;
    font-family: "Libre Baskerville", serif;
    color: #000000;
    text-align: center;
    text-transform: uppercase;
    text-shadow: -1px -1px 0 #166695, 1px -1px 0 #166695, -1px 1px 0 #166695, 1px 1px 0 #166695;
    margin-top: 5px;
}

.theme-fandomdesktop-light .MainPageHeading {
    color: #FFFFFF;
}

Oh I see that now. Now it's working. Thanks