I found this solution several years ago, I cannot find who the author was, but I give whom ever credit. It worked for me. However, every Steam update may or may not require an update of the procedure. Hope this helps.
"Steam font size
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Greetings,
I wrote a while back on how to increase the font size in the Steam platform. The trouble is, it reverts each time there is a Steam update.
If, like most of us, your gaming screen is in high resolution then the Steam fonts default size is ridiculously small. Currently, Steam doesn't give us any options to control this so it has to be done manually.
1) Exit Steam.
2) Browse your computer (or use the find tool) to go to this file:
C:\Program Files (x86)\Steam\resource\styles\steam.styles
3) Right click on the steam.styles file and open it with Notepad.
4) Use Notepad's find feature to look for "font-size=" and change each one to whatever font you'd like.
5) Save the file and restart Steam.
Now here is the new part. Since that file will revert to Steam's default one all the time, the only way to retain your settings is to use what Steam calls a Skin. This is a customized overlay of the Steam interface.
1) Right click the steam.styles file you just edited and choose copy.
2) Browse to the following directory:
C:\Program Files (x86)\Steam\skins
3) Create a directory with the name of your new Skin. It can be whatever you like, I just call mine Larger Fonts.
4) Underneath this new directory create new directories resources and then styles.
It should look like this: C:\Program Files (x86)\Steam\skins\Larger font\Resource\Styles
5) Paste your edited steam.styles into this styles directory.
6) Restart Steam.
7) From the Steam menu in the top left, go to Settings and then Interface.
8) Use the pull-down arrow under "Select the skin you wish Steam to use" and you should be able to choose Larger Font (or whatever you called your Skin).
9) Restart Steam and you should now have larger fonts which will remain through any updates. "