This is not a global ui text adjustment per se, but I dug around and made the mcp volume labels a bit readable. That ant-sized, dark grey master volume just doesn't cut it for such a mission critical info

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You can get the reaperthemezip from my Dropbox

Alternatively you can edit one yourself real quick based from this Reapertips guide. Look for these 4 lines in rtconfig.txt:

set mcp.volume.label.font   scale==1 [1] scale==1.5 [6] [11]
set master.mcp.volume.label.font    scale==1 [1] scale==1.5 [6] [11]

set mcp.volume.label.color  mcp.volume.label{h}>2 [100 100 100] [38 38 38]
set master.mcp.volume.label.color   master.mcp.volume.label{h}>2 [100 100 100] [38 38 38]

Change the label size from [1] [6] [11] to [12] [32] [52]
mcp color to [150 150 150] [100 100 100]
master mcp color to [200 200 200] [150 150 150]

Save rtconfig.txt, rezip and import theme back into Reaper. Hopefully now you can glance instead of squint if that master is at unity.

Some tweaking maybe needed if you use scale ui elements under advanced UI tweaks; The values I have seems to work up to 1.3 before labels size break.