My betta looks kinda sad :(?

Get him a bigger tank. The Marina Betta is only a 1/2 gallon tank, and while it is better than the average tiny betta tank, it is still pushing it toward small (1/2 gal is the minimum recommended size) and you need to be doing full water changes ever 4 days when using a tank this size. I looked at the tank, and it seems from the pics that there isn't much room with the plants for the betta to swim either. Generally bettas need 1 gallon per individual betta to be happy (so a single should be 1G, a split tank should be at 2 gal, etc). You should also have a water heater to maintain the temperature (especially in smaller volume tanks that are more easily heated and cooled by the environment) and a filter to keep the water clean. Bettas can be picky eaters and a lot don't prefer flakes. The best thing to get them is live, frozen or freeze dried food (like brine shrimp). This is sold at most pet stores that sell fish and aquatic specialty stores. If you live in a rural area and don't have access to this, you can use betta pellets. A lot of bettas don't like flakes, though.