Fix blurred tab close icon on FF61

[rafaelmardojai]

I don't know if this break something on previous FF versions.

[lunakurame]

Looks nice, thanks! This bug is in FF 59 too, but not FF 58. I'll merge this pull request tomorrow, because I'll be ending FF 58 support. Also your fix makes the whole button smaller - the button's border is too small.

Firefox 58:

Firefox 59+ with your fix:

But it's easy to fix, so that's okay.

.tab-close-button {
    padding: 2px;
    margin-left: 1px;
}

[rafaelmardojai]

Oh, i don't noticed that padding xD thanks.

BTW, i was thinking that we should add the --gnome-icons-hack-close-button-hover-bgimage and --gnome-icons-hack-close-button-hover-box-shadow variables to the themes, for better GTK look.

[lunakurame]

Merged your commits.

About those two new variables: if they are needed, then sure, we could add them. But are they needed? Everything looks fine to me on GNOME 3.18, I don't know about other versions.

[rafaelmardojai]

This is FF close tab button on hover (looks flat):

And this is GTK+ app close tab button on hover (3.26):

[lunakurame]

You're right. I added those two new variables and set them both to none. Now you can give them proper values for the 3.26 theme variants (I can't do it since I don't use 3.26).