feat(theme): add catppuccin themes with transparency and 256-color support

Add a theme package providing all four catppuccin flavors (mocha,
frappe, macchiato, latte) mapped to the xterm-256 palette. Themes are
selected with -theme or the CTOP_THEME environment variable; -i inverts
the flavor. Background slots use the terminal's native color for
transparency (opt out with -solid / CTOP_SOLID), and termbox is switched
to Output256 so colors render correctly. Replace hardcoded colors in the
compact and single views with ctheme portrait slots.

cwidgets/single/net.go

@@ -5,6 +5,7 @@ import (
 	"strings"
 
 	"github.com/bcicen/ctop/cwidgets"
+	"github.com/bcicen/ctop/theme"
 	ui "github.com/gizak/termui"
 )
 
@@ -26,13 +27,13 @@ func NewNet() *Net {
 	rx.Title = "RX"
 	rx.Height = 1
 	rx.Data = net.rxHist.Data
-	rx.LineColor = ui.ColorGreen
+	rx.LineColor = theme.Attr("net.rx")
 
 	tx := ui.NewSparkline()
 	tx.Title = "TX"
 	tx.Height = 1
 	tx.Data = net.txHist.Data
-	tx.LineColor = ui.ColorYellow
+	tx.LineColor = theme.Attr("net.tx")
 
 	net.Lines = []ui.Sparkline{rx, tx}
 	return net