I normally keep my Gospel Library app set to night mode, but I am not able to see the search highlights in night mode because the background and the highlights for search topics are almost the same color
I'm not exactly sure what you're talking about here, as they aren't all that close in color. In the night theme, the background is a near-black (#1d1d1d), whereas the search highlighter is a bright grey (#4a4444).
That is a different problem that I have already asked them to fix and the wont.
The version of Gospel Library you're using (the one built for Windows 8.1), will not be receiving anything beyond absolutely critical bug fixes for the foreseeable future. As Gospel Library for Windows 10 is on the verge of public release, version 2 of Gospel Library (the 8.1 version) will be put out to pasture shortly.
That is a different problem that I have already asked them to fix and the wont. So as a work around I tried to change back to the default mode and the app shows that it has changed but it does not actually change the way the text is displayed.
If you have auto theme turned on and are in an area that is even remotely dark, there's a good chance it's keeping it on the night theme for you. You'll need to turn off auto theme to switch the theme completely. Failing that, you could always close and reopen the app or even reinstall it.
I am using a Surface 3 Pro on Windows 8, and no I am not changing my operating system to Windows 10.
That's your choice, but you should know that you'll soon stop getting any updates. Not just in terms of app updates, but content updates, as the content source that the legacy app uses also won't be supported by the content team for much longer. This will mean no new manuals, no new editions of magazines, nor any fixes to broken or missing content. The numerous new features in Gospel Library for Windows 10 also will never be available to you.