Is there a way to change the font of all of the dimensions in a detail at once?
You can set the required font under Tools > Options > Document Properties
Select them all and change the font from the property manager.
Select them all, either with a box or by holding down Ctrl and selecting them individually, then go to the Other tab on the PropertyManager > Text Fonts, and un-check the "Use document font" box, then click the Font... button to select your new font.
What Deepak said is what you want for a global change. In the document properties side you will see the dimensions
and annotations text. In those selections you will see the settings for font style, color, size. Also keep in mind is if you want
this on drawings going forward you are going to want to make these changes to your drawing templates as well.
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