I don't think there is a way to create your own theme (or I don't know any). To change the size of all the text in the gallery I think the only way is selecting several text controls using CTRL and then change the size.
Another way of doing changes to affect all of the controls is making the size, colour or any property, related to one control that you could use as a reference (eg: Textbox input 2 Colour property= TextboxInput1.Colour). Then every time you change that reference it would change all the others. It could take time but if you want to play with the sizes and colours it could be worth it. Copy and paste the controls while you are building would help you with that because is going to keep that reference.
Of course, all this works if you do it when you start, not after...I think now using the CTRL for some cases is your only option