They're dull colors/textures with minimal borders, which is kinda ugly. That was popular in free browser RPGs fifteen years ago, so it might be a good choice if your game is deliberately riffing on those, but I don't think that's your intention here.
In the Battle Network series, tiles and buttons were simple, but careful color choices and some slightly ornamented borders did a lot to make them look good. MMBN Chrono X is mostly what I'm thinking of here, because it's fully displaced my memories of the originals. Here's an example, if you like: https://www.mmbnchronox.com/news/126
