There are many email appearance changes that you can make in AltaMail. Open the App Settings (View) - App Appearance section.
You can choose the default email font, size and color. This will be used for your new emails and replies to existing emails. This setting will also be used for incoming emails with no font formatting and plain text emails.

The "Dynamic Type" setting from the Device Settings App will apply to viewing emails. You can also use the 'Increase email text viewing size' to view emails with larger fonts to make reading easier.
You can also overide your device "Dynamic Type" setting in the App Appearance settings.

When you tap on 'Email Appearance' you will see a wide range of choices for the email inbox list layout. In the top section, tap on each part of the email list display to change the font, color and size of each text field. You can change the subject, sender and summary text.
Use the settings below to change other layout options such as the number of lines in the email summary and what buttons appear, eg. flag, star pinned etc. You can hide the contact icon image and change the color of unread emails. The account color can be displayed as a bar on the side or as the background of the email row.
You can change the email inbox background color and also the colors of the email thread messages for sent and received emails.

If you switch to Dark Mode the App Appearance settings will be different. When you edit settings while using Dark Mode those settings are only used for Dark Mode. Switching to Light Mode will use a different set of App Appearance settings.