Create a backup before doing any of this, in System > Backup > Download Backup: GENERATE ARCHIVE.
Update the list
Log into OpenWrt, go to System > Software: UPDATE LISTS.
If the update fails, check if you added DNS or not in Interfaces > your interface, e.g., LAN: EDIT > Advanced Settings > Use Custom DNS servers, e.g., 1.1.1.1 - This should fix the failure.
Install Luci-compat
Search "luci-compat" in the Filter and install it.
Install Luci-lib-ipkg
Repeat the same for Luci-lib-ipkg.
Install Argon theme files
Download this config file: luci-app-argon-config_0.9-20210309_all.ipk, then click on UPLOAD PACKAGE next to the update lists and install it.
Repeat the same for this file: luci-theme-argon, or you can try to grab the latest from the GitHub release page of Argon.
You should be able to see the theme now. If you don't, go to System > System > Language and Style > Design: Argon > SAVE & APPLY.
Try other themes
Go to System > Software > Filter: and type "theme." You will see a few options that can be installed with just one click. I personally use the Material theme.