This app was mentioned in 11 comments, with an average of 1.45 upvotes
It costs money that's the problem, "I don't recommend you to pirate" this application for a hard work just to get every icon but it is this one: https://play.google.com/store/apps/details?id=com.gmail.heagoo.apkeditor.pro
It replaces the core icon file of the app and replaces that with your choice of photo. Then saves it as a separate APK, which you then install. That newly created APK will have no traces of the original icon.
PS: I use APK Editor Pro hence cannot comment on the limitations of the free version.
In my opinion, your absolute best option for modding the APK is going to be APK Editor Pro ($4.99 USD) hands-down. I make a "Projects" folder in the apkeditor folder, then a sub-directory for a specified app. In this case, my custom build of AM2R (M2SR) that contains the files I use to modify the APK. What's more, is that APK Editor signs with testkey by default, but the user can manually import a keypair if desired.
I'd like to try this, too. What I would do to start is to change the package name and install a sort of cloned copy of the app, so as not to have to wipe my settings or anything during the experimentation process. The problem is, I don't know where widgets are located within the APK: it's tricky to search for, cause it seems that "widget" refers to different things in Android (not necessarily homescreen widgets).
I want to find the Reddit Is Fun widget and change the background to transparent and maybe make the font bolder, but the XML file I found (res/xml/ thread_appwidget_info.xml in the APK) only specifies dimensions, not color or font. So there must be another file I'm not seeing where the color and font are specified.
I should note that I'm trying to do this on my phone using APK Editor Pro app which also has a free version. IDK if you use Android Studio, but maybe you'd have better luck there...? Regardless, let me know if you figure anything out.
He brought the Pro version back to the Play Store a few weeks ago, but removed some of the functionality.
https://play.google.com/store/apps/details?id=com.gmail.heagoo.apkeditor.pro - I used this to change the name
Currently it's back in the playstore https://play.google.com/store/apps/details?id=com.gmail.heagoo.apkeditor.pro
There are several options depending on your skill set.
The first few I found:
There are more too. None of these are the one I used several years ago but they look like they can get the job done.