[cubic13]

The files involved are the following :

mcp_recarm_auto.png
mcp_recarm_auto_on.png
track_recarm_auto.png
track_recarm_auto_on.png

...without forgetting the instances of these that are also present in the different layouts subfolders. Basically, each file has three views of the involved control : the main one on the left, visible when no interaction occurs, the one in the middle that appears when the user hover the mouse cursor above it, and the one on the right, appearing when the control is clicked.

From which, you need an image editor (such as Paint.NET, Gimp, photoshop... - I use the former for 95% of my work, as I'm so at ease with it) which is able to edit PNG files and their transparency layer. The procedure is the following :

1) Make a copy of the CRado.ReaperThemeZip file in a safe place (outside the \ColorThemes subfolder of your Reaper installation).
2) Change its extension to .zip and unzip it : you should get both a CRado.ReaperTheme file and a CRado folder which contains the files listed above.
3) Edit the different instances of these files as wanted, with your image editor.
4) Rezip again BOTH CRado.ReaperTheme file and CRado folder together and change the resulting archive extension to .ReaperThemeZip.
5) Replace the existing theme with this new version in the \ColorThemes subfolder of your Reaper installation.

HTWH... If not, let me know with a screenshot the exact tint you would like for the "automatic record-arm" state of the record button.