feat(config): add theme customization (#81)

Added theme customization via config.toml
#17
```
[theme]
border = "#665C54"
accent = "#FABD2F"
muted = "#928374"
text = "#EBDBB2"
text_sel = "#FBF1C7"
match = "#FE8019"
desc = "#A89984"
desc_sel = "#EBDBB2"
sel_bg = "#504945"
scroll_info = "#83A598"
ghost_text = "#7C6F64"
hist= "#fe8019"
hist_sel = "#d65d0e"
sys = "#fb4934"
sys_sel = "#cc241d"
alias = "#b8bb26"
alias_sel = "#98971a"


```

---------

Co-authored-by: verse91 <versedev.store@proton.me>
Co-authored-by: VERSE <sunnyone0901@gmail.com>

internal/config/theme.go

@@ -0,0 +1,104 @@
+package config
+
+import (
+	"os"
+	"reflect"
+	"sync"
+
+	"github.com/BurntSushi/toml"
+)
+
+var defaultTheme = ThemeStyles{
+	Border:     "#a277ff",
+	Accent:     "#61ffca",
+	Muted:      "#6d6a7f",
+	Text:       "#edecee",
+	TextSel:    "#ffffff",
+	Key:        "#a277ff",
+	Match:      "#61ffca",
+	Desc:       "#9692a8",
+	DescSel:    "#edecee",
+	SelBg:      "#3d375e",
+	SelText:    "#110f18",
+	ScrollInfo: "#a277ff",
+	GhostText:  "#4B4A4C",
+	History:    "#1a2d36",
+	HistorySel: "#61ffca",
+	Sys:        "#1e1d28",
+	SysSel:     "#a277ff",
+	Alias:      "#2a2342",
+	AliasSel:   "#a277ff",
+}
+
+var (
+	themeMu     sync.RWMutex
+	themeStyles = defaultTheme
+)
+
+func Theme() ThemeStyles {
+	themeMu.RLock()
+	defer themeMu.RUnlock()
+	return themeStyles
+}
+
+type ThemeStyles struct {
+	Border     string `toml:"border"`
+	Accent     string `toml:"accent"`
+	Muted      string `toml:"muted"`
+	Text       string `toml:"text"`
+	TextSel    string `toml:"text_sel"`
+	Key        string `toml:"key"`
+	Match      string `toml:"match"`
+	Desc       string `toml:"desc"`
+	DescSel    string `toml:"desc_sel"`
+	SelBg      string `toml:"sel_bg"`
+	SelText    string `toml:"sel_text"`
+	ScrollInfo string `toml:"scroll_info"`
+	GhostText  string `toml:"ghost_text"`
+	Sys        string `toml:"sys"`
+	SysSel     string `toml:"sys_sel"`
+	History    string `toml:"hist"`
+	HistorySel string `toml:"hist_sel"`
+	Alias      string `toml:"alias"`
+	AliasSel   string `toml:"alias_sel"`
+}
+
+// LoadTheme reads theme.toml at filePath
+// Missing file - use defaults silently
+// Missing/empty fields - fall back to the default value for that field
+func LoadTheme(filePath string) {
+	themeMu.Lock()
+	defer themeMu.Unlock()
+
+	themeStyles = defaultTheme
+
+	data, err := os.ReadFile(filePath)
+	if err != nil {
+		// missing or unreadable file: use defaults
+		return
+	}
+
+	var t ThemeStyles
+	if _, err := toml.Decode(string(data), &t); err != nil {
+		return
+	}
+
+	applyThemeWithFallback(&themeStyles, t, defaultTheme)
+}
+
+// applyThemeWithFallback copies non-empty string fields from src into dst,
+// using def for any field that is empty in src
+func applyThemeWithFallback(dst *ThemeStyles, src, def ThemeStyles) {
+	dv := reflect.ValueOf(dst).Elem()
+	sv := reflect.ValueOf(src)
+	dv2 := reflect.ValueOf(def)
+
+	for i := range dv.NumField() {
+		sf := sv.Field(i).String()
+		if sf != "" {
+			dv.Field(i).SetString(sf)
+		} else {
+			dv.Field(i).SetString(dv2.Field(i).String())
+		}
+	}
+}