Use CSS variables rather than copying colors, spacing, etc. throughout.

[calvinmorett]

Hey there Ash, I saw your good first issue and wanted to contribute! 🧇

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• Updated onboarding.css, radio.css, toggle.css, and site.css to all use the same CSS variables: for Colors.
• Sorry for so many PRs!

:root{
    /* ### Theme Colors ### */
    /* Note: #14919b ! since this is used frequently, you could call this a theme color var */
    --theme_ashblu: #14919b; /* used in var --rad_label_i-chk */
    /* Note: #E5E5E5 ! is used frequently too */ 
    --theme_ashmercury: #E5E5E5; /* used in --feature_border */
    --theme_ashash: #333333;
    --theme_ashblu_darker: #0e7c86; /* DARKER THEME COLOR */
    --theme_ashblu_dark: #30919c; /* DARKER THAN THEME COLOR, LIGHTER THAN DARKER THEME COLOR */

    /* ### SRC/ONBOARDING ### */
    /* onboarding.css */
    --body_bg: #FAFAFA;
    --feature_bg: #FFF; /* can be used for rad label span / --rad_label_span */
    --feature_border: var(--theme_ashmercury); /* can be used for rad label input disabled / --rad_label_i-dis */

    /* ### SRC/OPTIONS ### */
    /* radio.css */
    --rad_label_i-chk: var(--theme_ashblu); /* can be used for rad label span box shadow / --rad_label_span_bs */
    --rad_label_i-dis: var(--feature_border);
    --rad_label_span: var(--feature_bg);
    --rad_label_span_bs: var(--rad_label_i-chk);

    /* toggle.css */
    /* Created Theme Variables */
    /* theme: _ashblue / _ashmercury */

    /* ### SRC/SHARED ### */
    /* site.css */
    /* Created Theme Variables */
    /* theme: _ashash / _ashblu_darker / _ashblu_dark */
}