Darken/Widen vertical line between the Tree Structure and message grid. Text colors?

I’m trying to increase contrast of a custom theme for a visually impaired user.

I would like to darken and widen the light gray vertical line between the tree structure and the message grid, but I can’t seem to find the reference in the code to the line or divider, only the padding.

Where might the “Mail, Calendar, Tasks, and Contacts” left column text colors be in the code?

Any advice appreciated.