Add theming for primary cursor and/or selection in themes that lack them

[Omnikar]

Some of the themes in runtime/themes make no distinction between primary and other cursors/selections; this, I think, should be addressed to prevent confusions like #1780 and in order to let people using said themes to make full use of the multiple cursors system.

List of themes that do not distinguish between primary and nonprimary cursors/selections:

• bogster
• ingrid
• rose_pine
• rose_pine_dawn
• serika-light *in making this list, I've realized that these two theme names mistakenly use hyphens instead of underscores
• serika-dark
• everforest_light
• everforest_dark

[CptPotato]

I agree with this, but maybe #1337 should be resolved first?

Since cursor.primary sets the color for both INS and NOR mode I didn't use it in my themes. Distinguishing between NOR/INS/VIS mode was more important to me.

[LollipopFt]

@Omnikar not sure if it's me, but gruvbox, dark_plus, monokai, onedark, spacebones_light, base16_default seem to have the same problem.

[Omnikar]

I just checked through those, and they seem to fall into two categories:

gruvbox, spacebones_light
These themes provide a theming for the primary cursor, but it is very similar to the normal cursor theme, and hard to notice.

dark_plus, monokai, onedark, base16_default
These themes don't provide a theming for the primary cursor, but do provide a theming for the primary selection, meaning you can distinguish cursors only if they have selections with them.