ref(colors): refine colors to match latest plugin updates

`barbar.nvim` had an update which separated buffer index colors from
their respective buffer separators colors. Additionally, `vimtex` had
an update which changed many names of its highlight groups.

This commit aims to relieve some of the visual oddities which may have
occured over the last few days, as a result of those changes.

colors/highlite.vim

@@ -665,10 +665,11 @@ local highlight_groups = {
 	gitrebaseSummary = 'Normal',
 
 	--[[ 4.3.35. Vimtex ]]
-	texMathGroup   = 'Number',
-	texMathSub     = 'Number',
-	texMathRegionX = 'Number',
-	texMathRegionY = 'Number',
+	texMathRegion   = 'Number',
+	texMathSub      = 'Number',
+	texMathSuper    = 'Number',
+	texMathRegionX  = 'Number',
+	texMathRegionXX = 'Number',
 
 	--[[ 4.4. Plugins
 		Everything in this section is OPTIONAL. Feel free to remove everything
@@ -729,11 +730,13 @@ local highlight_groups = {
 
 	--[[ 4.4.9. barbar.nvim ]]
 	BufferCurrent       = 'TabLineSel',
+	BufferCurrentIndex  = function(self) return {fg=self.InfoMsg.fg, bg=self.BufferCurrent.bg} end,
 	BufferCurrentMod    = {fg=tan, bg=black, style='bold'},
 	BufferCurrentSign   = 'HintMsg',
 	BufferCurrentTarget = 'BufferCurrentSign',
 
 	BufferInactive       = 'BufferVisible',
+	BufferInactiveIndex  = function(self) return {fg=self.InfoMsg.fg, bg=self.BufferInactive.bg} end,
 	BufferInactiveMod    = 'BufferVisibleMod',
 	BufferInactiveSign   = 'BufferVisibleSign',
 	BufferInactiveTarget = 'BufferVisibleTarget',
@@ -742,6 +745,7 @@ local highlight_groups = {
 	BufferTabpageFill = 'TabLineFill',
 
 	BufferVisible       = 'TabLine',
+	BufferVisibleIndex  = function(self) return {fg=self.InfoMsg.fg, bg=self.BufferVisible.bg} end,
 	BufferVisibleMod    = {fg=white, bg=gray_darker, style='italic'},
 	BufferVisibleSign   = 'BufferVisible',
 	BufferVisibleTarget = function(self)