LaTeX tufte-book class modifications

This is a project for a LaTeX expert with experience fine-tuning an [login to view URL] file for custom book designs. The task is to change the "[login to view URL]" file used by "[login to view URL]" on Overleaf, [login to view URL], to adopt the title page, Table of Contents (TOC), and Chapter/section headings of Tufte's Visual Display of Quantitative Information (VDQI). (The Overleaf template formats the front matter and section headings according to Beautiful Evidence (BE), another book by Tufte.) Specifically, the task is to adapt [[ Make Tuftian-style section headings and TOC formatting ]] section (at line 1531) to comport with VDQI Table of Contents formatting and numbered Chapter headings. I include a slightly modified definitions file, '[login to view URL]', which is a first-approximation of the TOC and handles the title page. But my use of '\lowercase' in line 1597 produces errors. A cleanly compilable template is the end goal.