I feed mine the same diet as my Russian. He is crazy for dandelions. I change the greens that they get fed weekly so they have a variety. Henry is a lot like my Russian in that he is a food hound and will overeat.
Oh and Henry doesn't seem to like Kale very much but that could just be his thing.
Marginated are part of the testudo group of tortoises. They cannot process the sugars in fruit and should not be fed it.
They often love things like strawberries and tomato, but it isn't good for them,
As a very occasional treat, buy yourself strawberries and then feed the tops (green , stalk and the tiny bit of fruit that's stuck to them) to your tort. You get the rest
I have read that anything with seeds should be limited. Henry only gets a piece of tomato once a week. I haven't even given him cucumber though. I give him strawberry tops too and he loves them. I also know he loves dandelion flowers.
Cucumber is nearly all water and has very limited nutritional value. It is more likely to cause runny poo.
Tomatoes can also cause runny poo. They have a high sugar content... they are just a red fruit... and should be fed only occasionally.
Red bell pepper is probably the best of the fruits if there is such a thing... But again feed infrequently.
A healthy diet for your marginated tort is green... Weeds preferably with maybe a pelleted diet once or twice a week and salad or veg leaves where weeds cannot be supplied.