For anyone who has avoided using some of these themes due to lack of contrast for selected text in the editbox or selected nicks in the nicklist, that should not be a deal-breaker.
The foreground/background colors for selected nicks, or selecting text in the editbox comes from the theme you defined in Windows' control-panel. This is also used for selected lines in menus or dialogs.
It's possible to edit just that 1 setting in windows without changing your entire theme. Or, you could tweak the mirc theme to choose editbox and listbox colors which contrast against the selection colors.
The way to change that setting will be different in different versions of windows. For Windows 7:
1. Open Control Panel
2. Choose Personalization
3. At the bottom of that area, click on "Window Color", which opens a dialog "Window Color and Appearance".
4. If you current theme is currently "unsaved theme", you'll either want to save the current theme to another name so you can return to it, or write down the current RGB colors before you change them.
5. In the 'item' dropdown, choose 'Selected Items'.
6. For now, let's avoid changing the font or the size, and notice the 2 squares labeled 'Color'. The top square is the background color and the bottom square is the text color. These control the text and background color for these selected items both here in mIRC and pretty much all Windows Apps.
7. You'll want to choose a background color that contrasts well against the foreground color you choose, but also contrasts well against the foreground/background colors you choose in the Alt+K dialog for the foreground/background colors in Editbox and Listbox.
8. When you click on either Color square, it opens a window showing a grid of 20 colors to choose from. If you don't like any of those shades, you can click 'Other' where you can create any RGB shade you wish.
9. When you click 'ok' or 'apply' the color setting is changed. It will then change your theme to be 'unsaved theme'. If you like it, you should click 'save theme' to give it a name. If you don't like this change, you could keep trying other color shades or just go back to your existing Control Panel Colors Theme prior to this change.