The graphics are very nice! There are few points that I could see being improved though:
The upgrade tree is a bit of a mess. You have light upgrades early on, but they serve no purpose early on (neither do they later on in the demo). I skipped them entirely. Afterwards, I found out that the Bomb upgrade was hidden there, which... makes no sense as far as the light upgrades go. Either it's in the wrong tree, or perhaps do not hide the upgrade (make it grayed out instead).
Likewise, I've skipped the middle tree (Battery Capcity vs All Weapon Damage) for a long time because it wasn't an important upgrade to me. Well, I found out that spending points on either one of those unlocks an entire new tree (Ore Magnet) with many new features (locked in the demo). You might want to not hide the tree or at least show the visible next upgrades but grayed out, which you do at the beginning, but not for all upgrades.
The game doesn't seem to work in Firefox, I had to switch to a Chronium-based browser. I would get this error in Firefox (maybe it was only me?):
Invalid binary data file size! (offset=15563428, size=18423317, file length=18647650)
The beginning of the game feels too slow. None of the upgrades I obtained had any real impact until quite some time. It took me a while before I destroyed my first block with something inside; I would run out of energy before I could reach or break it.
You usually have to do multiple runs before you can apply an upgrade. Most similar games tend to let you purchase at least one upgrade for each run so that you always feel like you're progressing. Here, I didn't feel like I was doing any progress at all. Sure I've increased my damage by 10% or by +10, but that changes nothing because I still need 2 hits to break the block. Most upgrades felt that way to me. Perhaps simply reducing the cost of the upgrades would help here.
Longer runs would also be nice too. Restarting all the time was a tad annoying. Perhaps the classic "I catch an item = gives me X extra more battery power" upgrade. Maybe it exists in the full game version? It always felt like I was rushing to mine for what felt like minimal rewards.
Overall, the game has good potential. The demo, however, kind of missed the goal. The game looks good, but there's nothing special going in the gameplay from what I could see. No cool mechanics, no progression "dopamine", it's just... slowly digging.