On the website font size is dependent on your browser. Check with your browser’s help manual, but normally “Zoom In” is “Cmd+” on Mac and “Ctl+” on Windows, while “Zoom Out” is “Cmd-” and “Ctl-” on Windows. The website currently does not support brightness or theme settings.
On the app, tap the “Aa” in the Comments bar when viewing an article. Then tap the “A” on the left to make text smaller and the “A” on the right to make text bigger. Use the slider to control brightness. The 3 circles at the top change the background color to white, sepia, or black; text color changes per theme selection.