Not possible, sadly, and it's one of the irritating theming limitations we have to deal with.
Few years ago, I remember having seen a post which was exposing a hack to get rid of the '%' sign. But, as it was involving the reaper.exe file itself, by changing something in the file header with a Hex editor, I didn't try to go further.
The post should still be somewhere in one of the subforums but I'm unable to retrieve it...
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DAW: Ryzen 3700X|Asus X-470Pro|32Gb|2 SSD512(M2)+1024|W10Pro(64)|RME Fireface UCX+ADA8200
Soft: Reaper|Cubase 10|Emulator X3 & several other VSTis (2 bridged)...
Gear: VMK-188+|MPD32|ME30P|Korg 05R/W|Roland D110|Yamaha TX802|Pre-MIDI stuff...
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