This plugin makes it easy to switch night mode on and off for custom themes.
Get the plugin here: feedmei/plugins.local at main · ltguillaume/feedmei · GitHub
Normally, the menu item to toggle night mode only controls the default tt-rss theme, no matter what theme you have selected. This plugin checks if the active custom theme has a themes.local/[theme]_night.css variant and enables/disables it. If there’s no such file, the plugin will revert to the default behavior (and use the default theme to toggle night mode).
An example of a theme that can make use of this plugin can be found here:
https://discourse.tt-rss.org/t/feedmei-a-clean-and-minimal-theme/2050
Alternatively, you can just rename or create a symlink to your favorite night mode theme with the name of your day theme +_night.css.