Icons are too small

[manake]

(Post removed.)

[eishiya]

The font Tiled uses is already customisable in Edit > Preferences > Theme. The default font seems to be inherited from your OS, which I think is sensible.

The Native theme inherits its colours from the OS I believe, but it doesn't do this very well on some OSes. I think this might be outside of Tiled's control, as it depends on Qt. The Tiled Fusion theme lets you specify a base colour and picks other colours appropriately, and I think generally does a good job with text contrast.
Icon contrast is still an issue, largely because Tiled uses coloured icons that can't be automatically adjusted. I personally like the coloured icons as I have an easier time telling them apart than monochrome ones (I recognise colour more readily than shape, even when I can see both very clearly). Tiled uses your OS's icon theme when it can, but I think on Windows, it always has to use its own. Providing different icon options (e.g. colourful vs monochrome, where the monochrome ones will take on the text colour) would be a good, inclusive solution, at the cost of requiring two+ icon sets. It might be possible to use a single icon set for both colourful and monochrome icons by assigning classes to the different elements that allow some elements (e.g. fills) to be hidden and the colours of the rest to be set to the text colour. This would require converting the remaining raster icons to vector, but I believe that is already a goal.

Larger icons and higher letter spacing might be beneficial for you, but would be problematic on lower-DPI screens, so those would need to be applied as part of the various DPI-sensitive changes, or need to be exposed as settings to the user. I think that a Preferences or toolbar right-click option to change the icon size would be nice, not just to aid for different DPI and vision levels, but also for temporary contexts like screen-sharing.

Related issues:
#3597 is about a new cross-OS theme, perhaps it could incorporate some of this, or at least be a higher-contrast default theme, instead of having Native as the default, which is variable in quality from OS to OS.
#3701 is about setting a custom stylesheet. While not a great solution to this issue since Tiled should be easy to see out of the box for the full range of reasonable DPIs, it would be a good solution for edge cases and unusual preferences.
#3678 is about some Qt elements being hard to see with certain theme colours, and not easily styled.