Search for element smallborderup:
<element name="smallborderup">
<style>
<set color="border3dhigh"/>
<drawline x1="0" y1="0" x2="WIDTH-1LS" y2="0"/>
<drawline x1="0" y1="0" x2="0" y2="HEIGHT-1LS"/>
<set color="border3dmid"/>
<drawline x1="0" y1="HEIGHT-1LS" x2="WIDTH-1LS" y2="HEIGHT-1LS"/>
<drawline x1="WIDTH-1LS" y1="0" x2="WIDTH-1LS" y2="HEIGHT"/>
</style>
</element>
You can see the bevel is build here, used colors: border3dhigh and border3dmid which are defined at the very beginning of the file:
<color id="border3dhigh" value="#00FFFFFF"/>
<color id="border3dmid" value="#808080FF"/>
Both colors are referenced from other elements, so maybe just add an own color definition.
smallborderup is used here on toolButton::MOUSE_OVER:
<element name="toolButton">
<style>
<property name="defaultSize" value="80 22" type="size"/>
<property name="defaultForegroundColor" value="#000000FF" type="color"/>
<property name="font" value="button" type="fontid"/>
<if state="ENABLED">
<if state="SELECTED">
<drawelem name="smallBorder" x1="0" y1="0" x2="WIDTH" y2="HEIGHT"/>
</if>
<if state="not SELECTED">
<if state="MOUSE_OVER">
<drawelem name="smallborderup" x1="0" y1="0" x2="WIDTH" y2="HEIGHT"/>
</if>
</if>
</if>
</style>
</element>
I just noticed, you can´t change the black and disabled text colors. So you need to manually textout with custom text colors in element toolButton this way:
<element name="toolButton">
<style>
<property name="defaultSize" value="80 22" type="size"/>
<set fontid="button"/>
<set colorproperty="foregroundColor"/>
<if state="ENABLED">
<set color="#0000FFFF"/> <!-- custom text color for disabled -->
<printtext x="25LS" y="HEIGHT/2" property="title" halign="left" valign="middle"/>
<if state="SELECTED">
<drawelem name="smallBorder" x1="0" y1="0" x2="WIDTH" y2="HEIGHT"/>
</if>
<if state="not SELECTED">
<if state="MOUSE_OVER">
<drawelem name="smallborderup" x1="0" y1="0" x2="WIDTH" y2="HEIGHT"/>
</if>
</if>
</if>
<if state="DISABLED">
<set saturation="30"/> <!-- change the saturation :) -->
<set color="#00FFFFFF"/> <!-- custom text color for disabled -->
<printtext x="25LS" y="HEIGHT/2" property="title" halign="left" valign="middle"/>
</if>
</style>
</element>








The bars are for measuring dimensions. I measured in Google Earth and then placed some supports at those same dimensions to keep its length accurate.