You don't necessarily need a theme for your thumbnails. There are YouTubers out there that use that, but think of it this way as well: it can start to get boring. Especially if the template isn't very strong and/or unique.
I would advice to use a more clear font. Something that can be seen easily. Use colors that fit each other and that make a nice showcase of each other. For example using a green text on a yellow background might not be so nice. Also try getting a stroke on your text, that makes it much more clear.
For your cutouts, you can try playing around with contrast and brightness. That can really amp up the quality of the image.
Of course Photoshop would be great for this, I'm not sure which other programs you could use though. Cooltext is a site that provides great text options though!