have you tried the tools>addons menu to change theme?

I have no idea of what to look for in the addons menu.

look for themes in left sidebar. that shows the available themes, such as light and dark

I am not the dean of harvard and have no concept of what you mean by context!!!!!!!!!

look for themes in left sidebar, that shows the available themes, such as light and dark. This is really an inadequate answer. my far left side bar has the following ONLY: chat, tasks, calendar, address book--that is it. the next column is my inbox, and the next is the INBOX with all the emails. After is the column that allows me to read the email contents.

Again, which sidebar are you referring to? Don't give up....just use a crayola.......b3 (not cubed because i can't find the superscript)

To repeat, - click tools>addons if using the menu bar, or click the hamburger icon in upper right and select addons there. - then, in the left sidebar of that pane, select 'themes' and available themes that are install will be on the right.

It would be useful to consult the KB. Namely https://support.mozilla.org/en-US/kb/install-themes-change-look-thunderbird