Hello, I'm currently using a brush and I want to set the minimum and maximum brush size, so no matter how hard I press, it never exeeds a certain size, and no matter how lightly I press, it never gets smaller than a certain size. Can you help me with this, and supply a screenshot if you can? I have another question I'd rather get out of the way here: How do you change the colour of the UI? it's currently dark/black is there a way to change it to white/gray? Thanks. Again screenshots to show me exactly where to go would be preferred. Thank you so much.

Brush size (you can see it on toolbar) is maximum size that your brush can be. You can change it in slider, via shift+drag or with [ and ] buttons.
To change minimal size you need to open brush settings (F5), check "Size" tab and set minimal size on curve. Example, on example you can see curve go from 50% to 100% so brush will never be smaller than 50% of maximum (min 50px in this case). If you want to choose minimal size in pixels or independent on maximum size, well, there is no way to do so.

To change program theme go Settings -> Themes