We have poor eyesight, so in vertical display mode, we could really benefit from having grid lines (at least horizontally) to visually distinguish each message. We have read various solutions on the Internet, but none seems to work at least not in dark theme mode, which is the default for a great many people with vision impairments (or just plain tired eyes). Perhaps some solutions that once worked have been uprooted, so to speak, during the course of Thunderbird updates. We are used to having grid lines in Outlook (desktop). We are currently exploring switching to Thunderbird, but it needs to support our vision needs. For us, this capability is fundamental, not optional. We are comfortable making config entries, creating and working in userChrome.css, and the like. We would be grateful for any guidance in this regard. Thank you kindly!