Personally I think the font for eg. the healthbar is way too streched out.
The hitcircle is a bit too big for the gameplay really distracting + with the thick sliderendcircle.
The opacity on the pause-, fail-, section pass- and section fail screen is a bit too bright and doesnt exist. In short: It hurts the eye. + the "Pause" word in the pause screen is overlapping with the "Continue" button. Maybe make the "Pause" smaller.
The ranking screen design is pretty good but the graph is misplaced.
The menu looks the worst in my opinion: The small ranking number is way too big (and streched and stuff). Try to create a red outline in the song-selection screen that fits the theme. Here is a good thread that can help you creating one
https://osu.ppy.sh/forum/t/612788 and the option-, mod and random button dont have symmetrical proportions.
I dont think I listed eveything in here but you should start working with that. Good Luck.
PS: maybe stick to one font at the same time.