feat: resolve css variables

[paviad]

Current style sheet logic ignores CSS variables, and can break when parsing them.

This PR properly resolves CSS variables.

[vitali-pr]

Hi @paviad, thank you for your contribution!

Would you mind signing the Contributor License Agreement and send back to us? The details are available at https://itextpdf.com/en/how-buy/legal/itext-contributor-license-agreement - it's required in order to merge the PR.

[paviad]

Signed, scanned and sent.

[denighte]

Hi @paviad, thank you very much for your idea and contribution, as I specified on your pull request in itext-core we had to fix it differently.

[paviad]

Can you provide a link to the comment? I can't find it. Thanks!

[denighte]

Can you provide a link to the comment? I can't find it. Thanks!

Oh, I actually posted in html2pdf repository instead, and this comment should have been in html2pdf, here is the link:
itext/itext-pdfhtml-dotnet#2 (comment)
Do note that the commit hash I posted there is actually for this repository, and in html2pdf there is only a single line change apart from tests