I made this custom style for my 98 year old grandma who struggled a lot every time yahoo mail made a redesign over the years. Based on gmail's already pretty basic html version, I hid all the elements she won't need leaving just the most basic email options, and made a couple of key buttons and links bigger like Send and Compose.

**NOTE: Before activating the style, go to https://mail.google.com/mail/?ui=html&zy=h and select "Always use basic html version" to make it persistent.

If you need to unhide some elements, look through the custom code and remove the relevent bits. It's pretty simple with mostly display:none elements and font-size increases.